vfio/mdev: Use vfio_init/register/unregister_group_dev

mdev gets little benefit because it doesn't actually do anything, however
it is the last user, so move the vfio_init/register/unregister_group_dev()
code here for now.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Liu Yi L <yi.l.liu@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Message-Id: <10-v3-225de1400dfc+4e074-vfio1_jgg@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>