Skip libvirt tests when libvirt not present.
Skip libvirt tests when libvirt not present.

Currently the libvirt tests don't make use of the test.skip_if decorator.
This should make it more obvious when the tests are not getting run.

We recently ran into an issue where libvirt tests were 'passing' in Jenkins
but they were actually not getting run, this should make things more clear.

Change-Id: Ied18d2403fc9accd6c6f739b08c9fe7cc3624bb2

n0ano/gantt,n0ano/gantt