Add support for using libvirt STONITH plugin in crowbar-pacemaker

For development purposes, it's very useful to be able to simply use the
external/libvirt STONITH plugin. It turns out that we only need to be
aware of the hypervisor IP address; all the other information we can
find through node attributes.

Useful resource:
http://www.hastexo.com/resources/hints-and-kinks/fencing-libvirtkvm-virtualized-cluster-nodes
Validation for libvirt
Add UI bits for libvirt mode