Adds support for stripped-down Ubuntu images which may not have kvm-ok in their path.

https://bugs.launchpad.net/juju-core/+bug/1342747
That `": "+` does not look right. The annotation comes before the error, like "$annotation: $error". I think you just want to pass `string(output)` straight in.

Can you please add a test for this case, to show what it looks like?