Change "None" in qvm_prefs

The string "none" is now changed to '' in qvm_prefs, and
only for certain properties (eg netvm).

qubesadmin/base.py

@@ -275,8 +275,6 @@ class PropertyHolder(object):
                 value = value.name
             if value is None:
                 value = ''
-            if str(value).lower() == "none":
-                value = ''
             try:
                 self.qubesd_call(
                     self._method_dest,

qubesadmin/tools/qvm_prefs.py

@@ -112,6 +112,9 @@ def process_actions(parser, args, target):
         args.property = args.property.replace('-', '_')
 
     if args.value is not None:
+        if str(args.value).lower() == "none":
+            if args.property in ["default_dispvm", "netvm", "template"]:
+                args.value = ''
         try:
             setattr(target, args.property, args.value)
         except AttributeError: