Add mixins to XModuleDescriptors as they are loaded during xml parsing

common/lib/xmodule/xmodule/x_module.py

@@ -576,10 +576,10 @@ class XModuleDescriptor(XModuleFields, HTMLSnippet, ResourceTemplates, XBlock):
         org and course are optional strings that will be used in the generated
             module's url identifiers
         """
-        class_ = XModuleDescriptor.load_class(
+        class_ = system.mixologist.mix(XModuleDescriptor.load_class(
             etree.fromstring(xml_data).tag,
             default_class
-        )
+        ))
         # leave next line, commented out - useful for low-level debugging
         # log.debug('[XModuleDescriptor.load_from_xml] tag=%s, class_=%s' % (
         #        etree.fromstring(xml_data).tag,class_))