virt-libirt: Fix two unittest failures

* When __init__ is called by the accessors.XMLElementNest factory
  the super() builtin has trouble referencing the class definition.

* Rather than a more complicated fix, I made these two cases just
  reference 'self.__class__' instead, which should remain correct
  since we shouldn't need to subclass those classes ever.

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