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

https://bugs.launchpad.net/juju-core/+bug/1342747
Yes,the signature of IsKVMSupported() should stay the same as it was.
The way I saw it working was that the implementation would exec.Command("kvm-ok") is it does now assuming it kvm-ok is on the path, and only in the case of error would it try again with an explicitly set path  as a fallback. ie exec.Command("/usr/sbin/kvm-ok")