more_used
bool (*more_used)(const struct vring_virtqueue *vq);
.more_used = more_used_split,
.more_used = more_used_packed,
.more_used = more_used_split_in_order,
.more_used = more_used_packed_in_order,
return VIRTQUEUE_CALL(vq, more_used);
if (!more_used(vq)) {