vagrant-libvirt support

This commits adds basic support to use the Vagrantfile with vagrant-libvirt 
plugin.
@danmcp @iNecas shouldn't we own this image, so we can add/remove/configure things inside, like a configure Docker the way we do with Virtualbox/AWS image, and other things? I'm also fine with having all this customization done in the shell script.

Next question is, who will maintain this image/provider? ;-)