vfio/mlx5: Refactor migration file state

Refactor migration file state to be an emum which is mutual exclusive.

As of that dropped the 'disabled' state as 'error' is the same from
functional point of view.

Next patches from the series will extend this enum for other relevant
states.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20221206083438.37807-7-yishaih@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>