[libvirt] block-backup API for libvirt

[Ian Main]

In qemu there will soon be drive-backup and block-backup commands that
allow you to create backups or point-in-time snapshots of VM images.
The original feature that drove this was to allow 'image fleecing',
which involves creating a light weight mechanism to take a point in time
snapshot of the running VM and access that remotely.

You can get a good idea of the API from this post:

http://lists.gnu.org/archive/html/qemu-devel/2013-10/msg02266.html

I would like to see this available via libvirt and wanted to start
working on a suitable API.  I'm curious if anyone has any input on how
they would like to see this done?  While I have done some work with and
on libvirt I'm no expert on the design philosophy behind the APIs so
some direction here before I get coding would be much appreciated.

>From what I can tell a volume based API that creates 'backups' and does
the NBD export call could work but I'd like to hear if that's
reasonable.

Thanks!

	Ian