Create by xpath

virt: New xml_utils.create_by_xpath() method + test
virt-libvirt: Descendent creation by accessors
virt-libvirt: libvirt_xml docstring update/fix
virt-libvirt: libvirt_xml module @staticmethod fix
virt-libvirt: Resolve libvirt_xml numa tune TODO
virt-libvirt: fix virsh_numatune test bug

TODO: Investigate numa_tune failures: <code>error: unable to set numa tunable: Unknown error 18446744073709551615</code>