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

https://bugs.launchpad.net/juju-core/+bug/1342747
I don't think we should be passing the path in here. This is leaking details about how we check if KVM is supported. I'd prefer if we dropped the comment above altogether, and then either:
 - just check in the (mockable) single location in IsKVMSupported (/usr/sbin/kvm-ok), or 
 - have IsKVMSupported call another non-exported function with the path to check.