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

https://bugs.launchpad.net/juju-core/+bug/1342747
This test needs to be split into either smaller tests or converted to use a table based approach. I'd probably plump for individual tests:
TestIsKVMSupportedNoPath
TestIsKVMSupportedExecReturnsError
TestIsKVMSupportedUserPath
(or choose your own names)

I'd also like to see a test that the default kvmPath() func returns "/usr/sbin" (trivial I know but a good check none the less as the chances of doing a typo in both places is low).