vfio/mlx5: Enable MIGRATION_PRE_COPY flag

Now that everything has been set up for MIGRATION_PRE_COPY, enable it.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20221206083438.37807-15-yishaih@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>