vfio/mtty: Enforce available_instances

The sample mtty mdev driver doesn't actually enforce the number of
device instances it claims are available.  Implement this properly.

Link: https://lore.kernel.org/r/162465624894.3338367.12935940647049917981.stgit@omen


Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>