[libvirt] the storage API requires capacity even using backingStore

Hi, I am using the storage API to create a new qemu image volume which has backingStore. It requires capacity to be set and uses the specified capacity as the size of the new volume. This causes issues if the specified capacity is not the same as the backingStore file's. It's not easy to get the capacity of the backing file in some cases. And qemu-img/kvm-img doesn't require a size option if it uses backing file. Is there any thing I could do to avoid this case? Or it needs a patch? Thanks a lot. BR UbuntuKids -- Ubuntu For Kids in China