Abstract all libvirt-related functions into separate class to allow replacing with e.g., RHEV

[ryran]

I wrote upvm with only virt-builder and local libvirt in mind. One of my colleagues pointed out that it would be pretty cool if it could spin up the new VMs in RHEV as well. The first step in that direction would be to extricate all libvirt-specific functions (virsh and virt-install and osinfo-query and all disk-related code) from the main flow and isolate them in a drop-in class that could easily be replaced. I can do that.

The next step would be writing that separate class to integrate with RHEV. I currently have no interest in that part. Will create a separate issue when the initial work is done.