Ensure old style images can be resized

In case an image without, or with an old cloud-init is booted, decide
if it's the file, or the nbd device that needs to be resized. Qcow2
files have to be mounted as nbd before attempting a resize.

Fixes bug 1212734

Change-Id: I29cbd4b334c39936342fa429bcc0b95378cade36
Merge "Ensure old style images can be resized"