* out_num: the number of sg readable by other side
* in_num: the number of sg which are writable (after readable ones)
* data: the token identifying the buffer.
- * Returns 0 or an error.
+ * Returns remaining capacity of queue (sg segments) or a negative error.
* @kick: update after add_buf
* vq: the struct virtqueue
* After one or more add_buf calls, invoke this to kick the other side.