Added support for configuring Kerberos authentication

This patch adds all the necessary bits to configure the ```kerberos``` authentication in ```libvirt```. Details about how to configure it:
* http://www.freeipa.org/page/Libvirt_with_VNC_Consoles
* http://wiki.libvirt.org/page/Libvirt_daemon_is_not_listening_on_tcp_ports_although_configured_to

It adds two new files to the module:
* ```/etc/sasl2/libvirt.conf``` provided by ```libvirt``` package.
* ```/etc/sasl2/qemu-kvm.conf``` provided by ```qemu-kvm``` package.

It requires #18 (it'll be rebased when the previous one is merged).
It also requires tests to be included.