Uses the "Matcher" interface of the testtools assertThat() call to
compare XML document strings safely. This will result in more
useful error results and will ignore attribute ordering issues
that caused problems with tests affected by lxml version skew.
Also converts test_libvirt_config to use XMLMatches, to
demonstrate its use.
Change-Id: I7821557a73eb8b5aca823cfccd02b4b660e0ffdb