vdpa/mlx5: Avoid using reslock in event_handler

event_handler runs under atomic context and may not acquire reslock. We
can still guarantee that the handler won't be called after suspend by
clearing nb_registered, unregistering the handler and flushing the
workqueue.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Message-Id: <20221114131759.57883-5-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>