vhost: implement vhost_vq_get_addr method

use vhost_vq_get_addr callback to get the vq address from backend

Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20200701145538.22333-10-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>