vfio/mlx5: Refactor to enable VFs migration in parallel

Refactor to enable different VFs to run their commands over the PF
command interface in parallel and to not block one each other.

This is done by not using the global PF lock that was used before but
relying on the VF attach/detach mechanism to sync.

Link: https://lore.kernel.org/r/20220510090206.90374-4-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>