virt-libvirt: Descendent creation by accessors

This allows using an XML accessor factory to create
an entire chain of element descendents if they don't
exist.  i.e. A disused property need not require
specific, or any XML to be in place.

Added unittest to also serve as an example for how this
can be used.
Signed-off-by: NChris Evich <cevich@redhat.com>