vfio/mlx5: Manage the VF attach/detach callback from the PF

Manage the VF attach/detach callback from the PF.

This lets the driver to enable parallel VFs migration as will be
introduced in the next patch.

As part of this, reorganize the VF is migratable code to be in a
separate function and rename it to be set_migratable() to match its
functionality.

Link: https://lore.kernel.org/r/20220510090206.90374-3-yishaih@nvidia.com


Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>