Make fakelibvirt python2.6 compatible

Fix bug #893757. In Python 2.6 xml.etree.ElementTree.fromstring
raises xml.parsers.expat.ExpatError instead of
xml.etree.ElementTree.ParseError when faced with invalid xml.
Adjust accordingly.

Change-Id: I1e4ae532e001618bded2b60e554915e5d6f7e038

nova/tests/fakelibvirt.py

@@ -15,7 +15,11 @@
 #    under the License.
 
 from xml.etree.ElementTree import fromstring as xml_to_tree
-from xml.etree.ElementTree import ParseError
+try:
+    from xml.etree.ElementTree import ParseError
+except ImportError:
+    from xml.parsers.expat import ExpatError as ParseError
+
 import uuid
 
 # Allow passing None to the various connect methods