tools: fix PropertyAction

self.default={} is mutable, so instead of modifying value derived from
that default, retrieve value, copy it and store again. Otherwise tests
(where the same parser is used multiple times) fails badly.
The same approach is used in argparse._AppendAction.

qubesmgmt/tools/__init__.py

@@ -71,7 +71,12 @@ def __call__(self, parser, namespace, values, option_string=None):
         except ValueError:
             parser.error('invalid property token: {!r}'.format(values))
 
-        getattr(namespace, self.dest)[prop] = value
+        properties = getattr(namespace, self.dest)
+        # copy it, to not modify _mutable_ self.default
+        if not properties:
+            properties = properties.copy()
+        properties[prop] = value
+        setattr(namespace, self.dest, properties)
 
 
 class SinglePropertyAction(argparse.Action):
@@ -102,8 +107,13 @@ def __init__(self,
 
 
     def __call__(self, parser, namespace, values, option_string=None):
-        getattr(namespace, self.dest)[self.name] = values \
+        properties = getattr(namespace, self.dest)
+        # copy it, to not modify _mutable_ self.default
+        if not properties:
+            properties = properties.copy()
+        properties[self.name] = values \
             if self.const is None else self.const
+        setattr(namespace, self.dest, properties)
 
 
 class VmNameAction(QubesAction):