nodedev_xml: extend MdevXML with attr elements

Add support for attribute elements of the mediated device
capability xml, e.g.:

```
 <attr name='assign_adapter' value='3'/>
 <attr name='assign_domain' value='43'/>
```

Signed-off-by: Sebastian Mitterle <[email protected]>

virttest/libvirt_xml/nodedev_xml.py

@@ -152,16 +152,42 @@ class MdevXML(CAPXML):
     """
     class for capability whose type is mdev
     """
-    __slots__ = ('type_id', 'uuid')
+    __slots__ = ('type_id', 'uuid', 'attrs')
 
     def __init__(self, virsh_instance=base.virsh):
         accessors.XMLAttribute('type_id', self, parent_xpath='/',
                                tag_name='type', attribute='id')
         accessors.XMLElementText('uuid', self, parent_xpath='/',
                                  tag_name='uuid')
+        accessors.XMLElementList(property_name='attrs',
+                                 libvirtxml=self,
+                                 parent_xpath='/',
+                                 marshal_from=self.marshal_from_attrs,
+                                 marshal_to=self.marshal_to_attrs,
+                                 has_subclass=False)
         super(MdevXML, self).__init__(virsh_instance=virsh_instance)
         self.xml = (' <capability type=\'mdev\'></capability>')
 
+    @staticmethod
+    def marshal_from_attrs(item, index, libvirtxml):
+        """Convert dictionary to an xml object"""
+        del index
+        del libvirtxml
+        if not isinstance(item, dict):
+            raise xcepts.LibvirtXMLError("Expected a dictionary of host"
+                                         " attributes, not a %s"
+                                         % str(item))
+        return ('attr', dict(item))
+
+    @staticmethod
+    def marshal_to_attrs(tag, attr_dict, index, libvirtxml):
+        """Convert xml object to a dictionary"
+        del index
+        del libvirtxml
+        if tag != 'attr':
+            return None
+        return dict(attr_dict)
+
 
 class StorageXML(CAPXML):