virtio-bus: introduce queue_enabled method

This patch introduces queue_enabled() method which allows the
transport to implement its own way to report whether or not a queue is
enabled.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20200701145538.22333-4-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>