[libvirt] virsh can't support VM offline blockcommit

Hi everyone! I use the libvirt(version: 1.2.2) and QEMU(version: 2.2.1) to test qemu snapshot features: I tried virsh blockcommit when VM offline, the virsh blockcommit failed: the error messase as below: error: Requested operation is not valid: domain is not running when I start the VM, the virsh blockcommit work fine! my question is : we need virsh support blockcommit when VM offline。how do？ we test qemu snapshot as follows： we need create snapshot of the cycle，and by virsh blockcommit merge snapshot chain，but whenVM offline， the virsh blockcommit will fail， someone suggest me：when VM offline，may use qemu-img commit to submit snapshot turn，but qemu-img can't modify snapshot chain backing relation，it need we use qemu-img rebase amend snapshot‘s backingfile， the main question is when we use qemu-img rebase modify snapshot's backingfile, if someone start the VM, it will may lead to VM filesystem readonly。 we shoule solve the question? thanks!