Add backup support for offline/shutdown domains about virtnbdbackup

[abbbi]

Another possible approach: if VM is offline, fall back to query the extends from the disk files directly via qemu-img map --output json /path/to/disk/image and save the data accordingly. qemu-img map takes care about existing overlay images, too:

"In particular, this commands dumps the allocation state of every sector of FILENAME, together with the top‐most file that allocates it in the backing file chain."

alternate: start qemu-nbd process for each attached disk and query exents.

would mean: in case domain is offline, stick to either copy or full backup only, no incremental supported here.

22:46 eblake: yes, but it doesnt give information wether if the extents are in the base or overlay image with --output json
22:46 eblake: or at least not in the version im using
22:47 which version of qemu-img are you using?
22:47 5.2.0, debian bullseye
22:48 hmm, yeah, that version of qemu-img map lacked the "present":true/false output for whether the image chain ends abruptly or would go deeper if a backing file were added to the
mix

[abbbi]

Fixed.