KVM/libvirt support with vanilla image definitions

Includes new image definitions without VirtualBox or VMware build steps added, so as to create vanilla OS images that can be booted under KVM.

----

Just so you get the gist of what's needed for https://github.com/puppetlabs/rspec-system/issues/18 more than anything.  The definitions are more or less just copies of the others that are there (except the Debian 7.0 one, which was from veewee).