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 not be necessary. `kvmtesting.TestSuite` embeds `juju/juju/testing.BaseSuite` embeds `juju/testing.CleanupSuite`.

(Also, whenever you embed a suite, you need to be sure to add the calls to SetUpSuite, SetUpTest, TearDownTest, TearDownSuite. If there's only one embedded suite then you can leave them off and it'll just use the that one's methods.)