[libvirt] [RFC] Image Fleecing for Libvirt (BZ 955734, 905125)

Hi all, QEMU-KVM BZ 955734, and libvirt BZ 905125 are about feature "Read-only point-in-time throwaway snapshot". The development is ongoing on upstream, which implements the core functionality by QMP command drive-backup. I want to demonstrate the HMP/QMP commands here for image fleecing tasks (again) and make sure this interface looks ready and satisfying from Libvirt point of view. We get cheap point-in-time snapshot, and export it through built in NBD server, by commands described below: 1. qemu-img create -f qcow2 -o backing_file=RUNNING-VM.img BACKUP.qcow2 (although the backing_file option is not honoured in the next step because we *override* backing file with an existing BlockDriverState, giving it here does no harm and also makes sure the created image is of right size.) 2. (HMP) drive_add backing=ide0-hd0,file=BACKUP.qcow2,id=target0,if=none (where ide0-hd0 is the running BlockDriverState name for RUNNING-VM.img) 3. (QMP) drive-backup device=ide0-hd0 mode=drive sync=none target=target0 (NewImageMode 'drive' means target is looked up as a device id, sync mode 'none' means don't copy any data except copy-on-write the point in time snapshot data) 4. (QMP) nbd-server-add device=target0 When image fleecing done: 1. (QMP) block-job-complete device=ide0-hd0 2. (HMP) drive_del target0 3. rm BACKUP.qcow2 Note: HMP drive_add/drive_del has no counterpart in QMP now but a new command blockdev-add to do similar things is WIP, which can be an alternative in QMP flavor. Any comments are welcome! -- Best regards, Fam Zheng