[Libguestfs] Why is virt-resize designed to involve two disks?

HI, As you've known, vhd-util and qemu-img both provide the capacity for resizing the original disk, but why is virt-resize designed to involve two disks, not only original disk? Is there any concern? Is it possible that only one original disk is involved in virt-resize? --  Regards,   Zhi Yong Wu