Return a new |_ContentTypeMap| instance populated with the contents
        of *content_types_xml*.

def from_xml(content_types_xml):
        """
        Return a new |_ContentTypeMap| instance populated with the contents
        of *content_types_xml*.
        """
        types_elm = parse_xml(content_types_xml)
        ct_map = _ContentTypeMap()
        for o in types_elm.override_lst:
            ct_map._add_override(o.partName, o.contentType)
        for d in types_elm.default_lst:
            ct_map._add_default(d.extension, d.contentType)
        return ct_map