libvirt: reworks configdrive creation

This refactor the creation of the configdrive by using
the same code schema as local/ephemeral/swap/disk.

Now this is the imagebackend that creates the configdrive like any other
disk attached to VM. This ensures that the configdrive file is created
where the imagebackend expect it.

This also removes the assumption that the configdrive was always created
at the right place, that was not true for rbd and lvm.

Related-Bug: #1361840

Change-Id: Ia03a83d593794c8b2a644360790cb15de9dea6e9
Merge "libvirt: reworks configdrive creation"