[sheepdog] confused comments of vdi_snapshot

[Yu Yang]

In sheep/vdi.c, the comments of function (line 1578)
	int vdi_snapshot(const struct vdi_iocb *iocb, uint32_t *new_vid)
says :
 * Both rollback and snap create started with current working VDI's snap_id + 1.
Working VDI
 * always has the highest snapid.

It seems that working VDI's snapid is always 0 , and has the highest vdi id (not
snapid) . 
Am I right?

Thanks.