virtio: fix error handling for debug builds

On error, virtqueue_add calls START_USE but not
END_USE. Thankfully that's normally empty anyway,
but might not be when debugging. Fix it up.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_ring.c

@@ -428,6 +428,7 @@ unmap_release:
 	if (indirect)
 		kfree(desc);
 
+	END_USE(vq);
 	return -EIO;
 }