vfio-mdev: turn VFIO_MDEV into a selectable symbol

VFIO_MDEV is just a library with helpers for the drivers.  Stop making
it a user choice and just select it by the drivers that use the helpers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Tony Krowiak <akrowiak@linux.ibm.com>
Link: https://lore.kernel.org/r/20230110091009.474427-3-hch@lst.de
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>