libvirt: make hypervisor configurable but only support kvm for now
libvirt: preliminary support for lxc targets and btrfs storage

NOTE: btrfs volumes require a patched libvirt, see
https://www.redhat.com/archives/libvir-list/2013-September/msg00530.html
Merge pull request #24 from saaros/libvirt_lxc

Libvirt: support for LXC containers