Comment 3 for bug 1270825

A possible design to elaborate comment #2.

1. source---> call pre_live_migration on destination
2. glance fetch returns ImageNotFound on destination
3. Handle exception and create disk, but keep it single layer qcow2. (ie. without a backing file)
4. return call back to source

5. Initiate live migration

Now in step 5, an incremental block copy would not work since the destination disk file would look like the source overlay file. We need to tell libvirt/kvm to do a deep copy. I don't know if this is possible with the current libvirt API. However, since qemu-img lets you rebase to any other backing file or flatten, I believe the mechanism exists.