support KVM for the buildserver

In order to finish the setup of our full staging environment of the buildserver on https://jenkins.debian.net, the full official build process must run in an KVM instance. Since the buildserver itself is a VM, that means running a VM in a VM, and for that only really KVM on KVM is feasible. VirtualBox and Xen do not support nested virtualization. That means ./makebuildserver and fdroid build need to be changed to work with KVM via libvirt. The work is almost complete, the last two pieces are:

• makebuildserver: replacing vagrant package with something that works with KVM, like libvirt snapshots
• fdroidserver/build.py: making a generic snapshot method that does bothVBoxManage snapshot and libvirt snapshots

@CiaranG could you describe that part of the workflow and process that is currently in use? Like how many snapshots are used on the buildserver? Are they reset, rebased, or recreated? When?