[libvirt] RFC: how to help to detect backing chain when no backing format info in images

[Nikolay Shirokovskiy]

I create transient domain with disk based on qcow2 image with 2 backing images.
I specify qcow2 format explicitly for the top image and omit the backing
chain in xml for create becase libvirt does not utilize this data anyway.
Top image and its backing file don't have backing file format information 
and format autodetection is off by default thus I get wrong backing chain
for the disk. Second image have format raw and there is no third backing
image obviously.

How can I get correct backing chain in domain xml in this case? I don't
want to set backing file format on this images because they are backup
images and I don't want to touch them in any way. Can we add option
to create API not to drop backing chain info that I can specify
on start? (Drop is forced on start in qemuDomainDetermineDiskChain)