Add libvirt-dev in anticipation of python-libvirt

python-libvirt now exists on PyPI. In anticipation of wanting to use it,
we need to ensure that libvirt-dev is installed on our slaves, so that
pip install python-libvirt will work.

Change-Id: I5958de644188e2745417a3cda319a2ea03896e7f
Merge "Add libvirt-dev in anticipation of python-libvirt"