This patch introduces new VhostOps vhost_vq_get_addr_op callback to get
the vring addr from the backend

Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20200701145538.22333-9-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>