TITLE: “FILLING DEVICE FOR FILLING CONTAINERS WITH A POURABLE PRODUCT” ***** TECHNICAL FIELD The present invention relates to a filling device for filling containers, in particular containers made of plastic or glass, for example bottles or flasks, or made of metallic material, for example cans, with a pourable product, preferably of a foodstuff type such as, for example, water, milk, fruit juices, carbonated or non-carbonated soft beverages, or the like. STATE OF THE ART Filling machines are known typically used for filling containers with a pourable product, which essentially comprise a carousel rotating around a vertical axis, a reservoir containing the pourable product and a plurality of valve filling devices, which are carried by the carousel at a radially peripheral portion thereof, are connected to the reservoir by means of respective circuits or ducts and are advanced by the carousel along a substantially circular path. These machines also comprise an inlet device, typically an inlet star wheel configured to sequentially feed empty containers to the carousel, and an outlet device, typically an outlet star wheel configured to receive containers filled with the pourable product from the carousel. In particular, the carousel receives a succession of empty containers from the inlet star wheel and directs the full containers towards the outlet star wheel. The carousel generally comprises a plurality of support elements each adapted to receive and maintain in a vertical position, below each filling device, a respective container to be filled. Each filling device is adapted to feed, while it is advanced along the aforesaid circular path by means of the rotary movement imparted to it by the carousel, a predetermined volume of pourable product to the relative container. Typically, the filling devices of the known type essentially comprise: - a hollow tubular body fixed to a peripheral portion of the carousel and internally defining a flow channel to feed the pourable product to a respective container to be filled located underneath the tubular body itself; - a shutter that engages the tubular body in a slidable manner and is movable inside the flow channel so as to selectively allow or prevent the outflow of the pourable product towards the respective container; and - an actuator configured to move the shutter inside the flow channel defined by the tubular body. Generally, the tubular body has a longitudinal axis parallel to the axis of the carousel and terminates at a lower end thereof with an axial discharge opening fluidly communicating, in use, with an end opening defined by an upper edge of the respective container to be filled. The flow channel extends coaxially to the axis of the tubular body and comprises one segment of constant section, usually cylindrical, and at least one segment of variable section, usually truncated-conical, positioned upstream of the discharge opening and narrowing in the direction of the latter, up to a section of minimum diameter. The shutter, which typically coaxially engages the flow channel, is axially movable inside the flow channel between: - a lower closing position, in which the shutter seals shut the segment of variable section of the channel, so as to interrupt the flow of the pourable product through the discharge opening; and - an upper opening position, in which the shutter delimits together with the segment of variable section an annular passage fluidly communicating with the discharge opening, so as to allow the flow of the pourable product towards the end opening of the respective container. Typically, the shutter comprises a main body, usually substantially cylindrical in shape, and an interaction portion, for example piston-shaped, extending axially from the main body in the direction of the discharge opening and configured to abut against the segment of variable section of the flow channel when the shutter is in the closing position. In detail, the interaction portion is adapted to close in a fluid-tight manner the aforesaid section of minimum diameter, when the shutter is in the closing position. Actuators of the pneumatic or hydraulic or oleodynamic type are known. The actuators of this type, however, are not suitable for the aseptic applications, as they require a complicated separation between the "dirty" actuation components (which must absolutely remain outside the flow channel) and the "clean" components (for example, the shutter) inside the flow channel. In order to overcome this drawback, actuators of the magnetic or electromagnetic type have been widely used: they comprise a driving member outside the flow channel and a driven member integral with the shutter and therefore inside the flow channel. The driven member is usually defined by permanent magnets. The driving member may be defined by: - permanent magnets; in this case, an axial movement of the driven member will correspond to an axial movement of the driving member, thanks to the magnetic interaction between the two members; or by - an electrically energizable solenoid; in this case, depending on the electric current supplied to the solenoid, an axial movement in one direction or the other of the driven member, and therefore of the shutter, will be determined, thanks to the electromagnetic interaction between the two members. In this manner, the movement of the shutter is obtained without compromising the sterility and/or asepticity of the filling. The need to center the shutter inside the flow channel is known, i.e., to maintain coaxiality of the shutter to the flow channel during the movement between the opening position and the closing position, namely during its stroke. This need is particularly felt in the case of magnetic or electromagnetic actuators, wherein the shutter is mechanically disconnected from the rest of the actuator. In this regard, it is known to provide the shutter with radial guide elements adapted to axially slide on an inner wall of the tubular body delimiting the flow channel. In detail, these guide elements are arranged in the main body of the shutter and radially protrude from it at two end portions of the shutter axially opposite to each other and having, due to the presence of the guide elements, an external diameter substantially equal to the internal diameter of the flow channel. Therefore, the guide elements keep the shutter centered inside the flow channel during its stroke, by means of sliding contact with the inner wall of the flow channel. However, the guide elements work by sliding and are thus subject to friction and to wear (as well as the aforesaid inner wall). In some cases, the guide elements are provided with a thin coating, such as for example, Diamond-Like-Carbon (DLC) or chromium nitride, in order to increase the surface hardness and reduce the sliding friction. However, the presence of the coating increases the total costs and increases the risks of contamination of the pourable product, as even microscopic parts of the coating may progressively detach from the guide elements, affecting the asepticity and/or the sterility of the filling process. Furthermore, the coating may adversely impact the magnetic and/or electromagnetic coupling between the driving member and the driven member. OBJECT AND SUMMARY OF THE INVENTION An object of the present invention is to produce a filling device for filling containers with a pourable product which is of high reliability and limited cost, and allows to overcome at least some of the drawbacks specified above and associated with filling devices of the known type. According to the invention, this object is achieved by a filling device as claimed in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, a preferred non-limiting embodiment thereof is described in the following, purely by way of example and with the aid of the accompanying drawings, wherein: - Figure 1 is a schematic top view, with parts removed for clarity, of a filling machine comprising a plurality of filling devices produced according to the present invention; and - Figures 2 and 3 are partially sectioned schematic side views, on an enlarged scale and with parts removed for clarity, of a filling device during two distinct operating conditions. DETAILED DESCRIPTION With reference to Figure 1, number 1 indicates as a whole a filling machine configured to fill containers 2 with a pourable product, preferably of the foodstuff type, such as for example, water, fruit juices, milk, carbonated or non-carbonated beverages, wine, beer or the like. The containers 2 may be defined, for example, by bottles or flasks made of plastic or glass or by cans made of metallic material (aluminium). The machine 1 comprises: - a carousel 3 rotatable around an axis X (preferably vertical) and having a plurality of peripheral receiving seats 3a for carrying respective containers 2; - a reservoir (not illustrated) for containing the pourable product; - an inlet star wheel 4 for feeding containers 2 to be filled to the carousel 3; - an outlet star wheel 5 for withdrawing the filled containers 2 from the carousel 3; and - a plurality of valve filling devices 6, carried peripherally by the carousel 3 so as to overlap the seats 3a, each fluidly connected to the reservoir by means of a duct (not illustrated) and each configured to dose a predetermined amount of pourable product to a respective container 2 carried by the underlying seat 3a. In particular, each filling device 6 is selectively activatable to control the outflow of the pourable product towards a respective container 2 carried by the carousel 3 and placed vertically below the device 6 itself. For the sake of simplicity, reference will be made in the following to a single filling device 6. However, the structural and functional characteristics described for this filling device 6 are equally applicable to each filling device 6 of the machine 1. With reference to Figures 2 and 3, the filling device 6 comprises: - a tubular body 7 having a longitudinal axis A, internally defining a flow channel 8 for the pourable product, comprising an inner longitudinal wall 8a delimiting the channel 8, and terminating at one end thereof with a discharge opening 10 for feeding the pourable product towards a respective container 2; and - a valve member, or shutter 11, engaging the channel 8 and axially movable therein between an opening position (Figure 3), in which the shutter 11 allows a flow of the pourable product through the discharge opening 10, and a closing position (Figure 2), in which the shutter 11 prevents the flow of the pourable product through the discharge opening 10; and - an actuator 12 of the magnetic or electromagnetic type configured to control the movement of the shutter 11 inside the channel 8. In detail, the tubular body 7 is substantially cylindrical and the axis A is preferably vertical and parallel to the axis X of the carousel 3. The tubular body 7 also has an inlet opening 13 arranged on an axially opposite side with respect to the discharge opening 10, through which the channel 8 receives the pourable product from the reservoir. In the case illustrated, both the inlet opening 13 and the discharge opening 10 are axial. Conveniently, the channel 8 extends coaxially to the axis A and comprises: - a segment of constant section, substantially cylindrical and provided with the inlet opening 13; and - a segment of variable section 8b substantially truncated and tapering towards the discharge opening 10, up to a section of minimum diameter corresponding to the discharge opening 10 itself. Hence, the segment of variable section 8b is arranged downstream of the segment of constant section with respect to a flow direction of the pourable product inside the channel 8. The shutter 11 is arranged coaxially to the axis A inside the channel 8 and comprises a main body 11a, having a substantially cylindrical shape, and an interaction portion 11b, for example shaped like a piston, extending axially from the main body 11a in the direction of the discharge opening 10 and configured to abut against the segment of variable section 8b and close the latter in a fluid-tight manner. More precisely, the interaction portion 11b comprises a sealing element, preferably a sealing ring 14 (for example, an O-ring) made of an elastomeric material, fixed to an axial end of the interaction portion 11a facing the discharge opening 10 and intended to selectively cooperate with the segment of variable section 8b, in order to seal it in a fluid-tight manner and prevent the passage of pourable product towards the discharge opening 10. For this purpose, the shutter 11 is axially movable inside the channel 8 between: - the opening position, in which the interaction portion 11b, in particular the sealing ring 14, delimits together with the segment of variable section 8b an annular passage communicating fluidly with the discharge opening 10, so as to allow the flow of the pourable product towards the container 2; and - the closing position, in which the interaction portion 11b cooperates in a fluid-tight manner with the segment of variable section 8b, so as to interrupt the flow of the pourable product through the discharge opening 10. In the case illustrated, the actuator 12 is of the magnetic type and comprises a driving member 15 located outside the channel 8 and a driven member 16 integral with the shutter 11 and configured to magnetically interact with the driving member 15. In particular, the driven member 16 comprises permanent magnets incorporated in the main body 11a of the shutter 11 at a central portion thereof. The driving member 15 surrounds this central portion, is controllable to be axially moved and comprises permanent magnets radially facing the permanent magnets of the driven member 16 with respective magnetic poles of opposite sign to each other. In this manner, an attractive magnetic force is determined between the driving member 15 and the driven member 16, whereby an axial movement of the driven member 16 will correspond to an axial movement of the driving member 15 and, therefore, of the shutter 11 inside the channel 8, thus determining the selection between the opening position and the closing position. In an alternative embodiment not illustrated, the driving member 15 comprises permanent magnets organized according to a Halbach array configuration, of the type described and illustrated in the International Application No. WO-A-2021121593, in the name of the same Applicant. In a further alternative embodiment not illustrated, the actuator 12 is of the electromagnetic type. In this case, the driving member comprises an electrically energizable solenoid. The shutter 11 will be moved to open or close depending on the direction of the electric current. According to the invention, the filling device 6 further comprises at least one magnetic linear bearing configured to axially guide and radially support the shutter 11 in levitating manner with respect to the inner wall 8a of the channel 8. Conveniently, the at least one linear bearing comprises a first magnetic linear bearing 17b arranged in a position proximal to the discharge opening 10 and distal with respect to the inlet opening 13, and a second magnetic linear bearing 17a arranged in a position proximal to the inlet opening 13 and distal with respect to the discharge opening 10. In this manner, the first bearing 17b and the second bearing 17a are configured to center the shutter 11 inside the channel 8 during its axial movement between the opening position and the closing position. In practice, each magnetic linear bearing 17a or 17b is configured to keep the shutter 11 at a non-zero distance from the wall 8a and to keep the shutter 11 centered with respect to the axis A, i.e., to keep it coaxial and/or aligned with respect to the axis A. From the foregoing, it can be seen how the magnetic linear bearings 17a and 17b define contactless axial guide elements for the shutter 11. For simplicity, reference will be made in the following to a single magnetic linear bearing 17a or 17b. The magnetic linear bearing comprises: - a first magnetic element 18 fixed to the tubular body 7 and arranged externally to the channel 8; and - a second magnetic element 19 which is fixed to or integral with the shutter 11, is arranged inside the channel 8, and magnetically interacting with the first magnetic element 18, so as to cause magnetic levitation of the shutter 11 with respect to the wall 8a. In particular, the first magnetic element 18 and the second magnetic element 19 are each defined by a respective permanent magnet that extends annularly around the axis A and is radially biased to have a first radially inward magnetic pole and a second radially outward magnetic pole of opposite sign to the first magnetic pole. According to the invention, the first magnetic element 18 and the second magnetic element 19 are arranged facing each other radially with respective magnetic poles of equal sign, so as to determine a magnetic repulsion force and cause said magnetic levitation of the shutter 11 with respect to the wall 8a. Therefore, the aforesaid first inner magnetic pole of the first magnetic element 18 has an equal sign to the aforesaid second outer magnetic pole of the second magnetic element 19. A magnetic equilibrium force is therefore determined which allows the shutter 11 to be radially supported and axially guided without any contact with the wall 8a. The shutter 11 is movable between the opening position and the closing position by an axial stroke of predetermined length. Advantageously, the first magnetic element 18 has an axial extension at least equal to said stroke. In particular, the second magnetic element 19 has an axial extension smaller than the axial extension of the first magnetic element 18, so as to remain completely radially surrounded by the first magnetic element 18 for the entire length of said stroke. In this regard, comparison is made between Figure 2 (closing position) and Figure 3 (opening position). In this manner, the radial support and axial guiding of the shutter 11 are ensured for the entire length of the stroke itself. According to a further aspect of the invention, the filling device 6 comprises a first ferromagnetic element, for example a ferromagnetic washer 20 which extends annularly around the axis A. The washer 20 is fixed to the tubular body 7 and is axially interposed between the discharge opening 10 and the first magnetic element 18 of the aforesaid first magnetic linear bearing 17b. The washer 20 is configured to magnetically interact with the second magnetic element 19 of the first magnetic linear bearing 17b and attract it to itself to bias a displacement of the shutter 11 towards the closing position. Actually, the second magnetic element 19 attracts the washer 20 to itself, which is, however, fixed. The result is a displacement of the second magnetic element 19, and therefore of the shutter 11, towards the washer 20 itself. Thanks to the peculiar positioning of the washer 20, the shutter 11 tends to close the discharge opening 10. In other words, the interaction between the washer 20 and the second magnetic element 19 defines a magnetic spring that tends to close the valve. This configuration is particularly advantageous in the case where the actuator 12 is of the electromagnetic type: it is in fact possible to keep the shutter 11 in the closing position without excessive use of electrical energy. Advantageously, the filling device 6 comprises, in combination with or as an alternative to the washer 20, a second ferromagnetic element, for example a second ferromagnetic washer 21 which extends annularly around the axis A. The second washer 21 is fixed to the tubular body 7 and is axially interposed between the inlet opening 13 and the first magnetic element 18 of the aforesaid second magnetic linear bearing 17a. The second washer 21 is entirely similar to the first washer 20, except that the magnetic spring defined by the interaction between the second washer 21 and the second magnetic element 19 of the second magnetic linear bearing 17a tends to open the valve, i.e., tends to hold the shutter 11 in the opening position. It is in this manner possible to perform, for example, a cleaning operation ("CIP") of the filling device 6. This configuration is particularly advantageous in the case where the actuator 12 is of the electromagnetic type: it is in fact possible to keep the shutter 11 in the opening position without excessive use of electrical energy. In an embodiment not illustrated, the first washer 20 and/or the second washer 21 are arranged internally to the channel 8, respectively downstream of the bottom dead center and upstream of the top dead center of the relative second magnetic element 19. Conveniently, the first washer 20 and/or the second washer 21 are arranged externally to the channel 8. In this manner, the fluidic interference of the pourable product within the channel 8 is limited. Advantageously, the filling device 6 comprises at least one Hall effect position sensor 22, which is arranged at the at least one magnetic linear bearing, for example at the second magnetic linear bearing 17a, in a position radially and/or axially proximal to the first magnetic element 18 of the latter. The Hall effect sensor 22 is configured to detect an axial position of the shutter 11 inside the channel 8 by means of the magnetic interaction of the second magnetic element 19 with the first magnetic element 18 during movement of the shutter 11 between the opening position and the closing position. It is thus possible to exploit the presence of the magnetic linear bearings 17a, 17b in order to know the axial position of the shutter 11, thus avoiding complicated detection systems and decreasing the components and the total costs of the filling device 6. The first ferromagnetic element 20 may be considered an upper ferromagnetic ring. The second ferromagnetic element 21 may be considered a lower ferromagnetic ring. From an examination of the characteristics of the filling device 6 produced according to the present invention, the advantages that it allows to obtain are evident. In particular, the presence of the magnetic linear bearings 17a, 17b makes it unnecessary to use contact guide elements of the type described above to keep the shutter 11 aligned and/or centered with respect to the channel 8 during its movement. In this manner, it is possible to reduce (and in particular eliminate altogether) the friction between the shutter 11 and the tubular body 7. It is no longer necessary to provide a coating of the type mentioned above, which, in addition to being expensive and possibly detrimental to the asepticity/sterility of the filling process, may interfere with the magnetic or electromagnetic field of the actuator 12 and, therefore, with the control of the shutter 11. It is clear that modifications and variations may be made to the filling device 6 described and illustrated herein without thereby departing from the scope defined by the claims.