vfio/mdev: Use a module softdep for vfio_mdev

Use an explicit module softdep rather than a request module call such
that the dependency is exposed to userspace.  This allows us to more
easily support modules loaded at initrd time.

Reviewed by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>