qubes module: Typo fix

This commit fixes a typo, where the exception type DontSave was used,
even though the name of the function dontsave was intended.

qubes/__init__.py

@@ -429,7 +429,7 @@ def stateless_property(func):
     exposed through management API (including qvm-prefs etc)'''
     return property(func.__name__,
         setter=property.forbidden,
-        saver=property.DontSave,
+        saver=property.dontsave,
         default=func,
         doc=func.__doc__)