vm: fix placeholder methods signatures in Features class

Some methods inherited from dict (pop and setdefault here) are covered
by placeholders raising NotImplementedError. Lets fix their signatures
(to match those of dict) to really get NotImplementedError, instead of
TypeError.

qubes/vm/__init__.py

@@ -108,7 +108,7 @@ class Features(dict):
         for key in self:
             del self[key]
 
-    def pop(self):
+    def pop(self, _key, _default=None):
         '''Not implemented
         :raises: NotImplementedError
         '''
@@ -120,7 +120,7 @@ class Features(dict):
         '''
         raise NotImplementedError()
 
-    def setdefault(self):
+    def setdefault(self, _key, _default=None):
         '''Not implemented
         :raises: NotImplementedError
         '''