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: Chris Evich <cevich@redhat.com>
virt-libvirt: libvirt_xml docstring update/fix

Signed-off-by: Chris Evich <cevich@redhat.com>
virt-libvirt: libvirt_xml module @staticmethod fix

All @staticmethods on LibvirtXMLBase subclasses must
accept and pass a virsh_instance parameter, but many
were not.

Signed-off-by: Chris Evich <cevich@redhat.com>
virt-libvirt: Resolve libvirt_xml numa tune TODO

Simplify access to numa parameters by making it
into a propery to return attributes as a dict.

Signed-off-by: Chris Evich <cevich@redhat.com>
virt-libvirt: fix virsh_numatune test bug

Test was failing when start_vm was "no" but gest was running.
Test was failing when requested numaset greater than available
on host.  Test was failing when --config parameter was used
but guest was not restarted.  Many thanks to Yang Dongsheng
<yangds.fnst@cn.fujitsu.com> for his help finding and helping
fix some of these problems.

Signed-off-by: Chris Evich <cevich@redhat.com>