mnaio uses LVM as default VM disk backend but that
requires a lot of available disk space.
A alternative option, the qcow2 file based backend
is added to take benefit of thin provisioning.
This backend can be triggered by setting the override
`default_vm_disk_mode` to `file`
Change-Id: Iaf97fef601f656901b6913eaafb9a6c28d4b7ba6