Comment 16 for bug 1130146

The nova.tests.matchers.XMLMismatch class implements the get_details() method returning a dictionary with the expect
and actual XML output. It is returning a string values, but the testtools code requires that the dictionary values are instances of the testtools.content.Content object. To quote the 'get_details' docs on testtools.matchers.Mismatch

:return: a dict mapping names to Content objects. name is a string to
name the detail, and the Content object is the detail to add
to the result. For more information see the API to which items from
this dict are passed testtools.TestCase.addDetail.

I've posted https://review.openstack.org/#/c/22350/ to fix this