xmlbuilder: add a __repr__ for XMLBuilder object

To help with debugging.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

virtinst/xmlbuilder.py

@@ -1168,3 +1168,6 @@ class XMLBuilder(object):
             elif key in childprops:
                 for obj in util.listify(getattr(self, key)):
                     obj._add_parse_bits(node)
+
+    def __repr__(self):
+        return "<XMLBuilder %s %s>" % (self._XML_ROOT_NAME, id(self))