Add functions to support configure interrupt.
The configure interrupt process will start in vhost_dev_start
and stop in vhost_dev_stop.

Also add the functions to support vhost_config_pending and
vhost_config_mask, for masked_config_notifier, we only
use the notifier saved in vq 0.

Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20211104164827.21911-8-lulu@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>