Add 9p (virtfs) support to vagrant-libvirt.

This is a follow up of this issue https://github.com/pradels/vagrant-libvirt/issues/7.
I first patched libvirt to be able to attach filesystems:  http://libvirt.org/git/?p=libvirt.git;a=commitdiff;h=0778fc1ab9c39bb989d168ec44a980c3db943fca and then started working on vagrant-libvirt integration. I also used code from https://github.com/adrahon/vagrant-kvm/pull/125/files .
So now you can set :type => "9p" for your shared folders and they will mount as such.

This patch upgrades vagrant-libvirt Gemfile to be compatible with vagrant 1.5 (you don't need to use require_plugin in your Vagrantfile). 

I am not sure it is ready yet for integration so it's more an RFC request. It works on my machine though. I can also alternate with NFS. I've removed the rsync implementation since Vagrant provides one now. I've not tested rsync though. I was wondering if there was a way to automatically test it (via vagrant-spec maybe ?)