vhost: introduce new VhostOps vhost_dev_start

This patch introduces new VhostOps vhost_dev_start callback which allows the
vhost_net set the start/stop status to backend

Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20200701145538.22333-7-lulu@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>