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

https://bugs.launchpad.net/juju-core/+bug/1342747
Hey, you make good points. But we are breaking encapsulation (information hiding) forcing the caller to know where to look. ie we are exposing an unnecessary implementation detail. The caller doesn't, nor shouldn't, know the detail of how kvm support might be determined. If kvm-ok were to move to a different path, the implementation should be able to be changed without the caller having to know, which would not be the case with the suggested implementation. Hence there's unnecessary coupling and leakage of implementation detail. The IsKVMSupported() method is akin to a library api call rather than a piece of higher level business logic which would benefit from dependency injection to plugin the moving parts.