Things like Guest.Seclabel, VirtualDevice.VirtualAlias, etc.
Now we don't need to track a full xpath with each class, just its root
name and we build its hierarchy depending on its parent.
This allows to make use of libvirt network filtering support with virt-install.
With the additional option "filterref" in the --network parameter one can
configure any defined nwfilter per network interface, i.e.:
virt-install ... --network network=mynet,model=virtio,filterref=clean-traffic
(crobinso: add an xmlparse test case)
