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 should be moved so that the comment is attached to the right thing.

Also, it's more typical in the code base to use a `var` in this case, and then use the suite's `PatchValue` method. In export_test.go, you'd have something like:

```go
var KVMPath = &kvmPath
```

And then in your test code you can just do:

```go
s.PatchValue(kvm.KVMPath, "")
```