tools/qvm-prefs: fix printing None value

Don't print None value as 'None' string, but as empty one (same as at
API level). Otherwise it is indistinguishable from VM named 'None', or
same string property value.

qubesadmin/tools/qvm_prefs.py

@@ -126,7 +126,9 @@ def process_actions(parser, args, target):
         return 0
 
     try:
-        print(str(getattr(target, args.property)))
+        value = getattr(target, args.property)
+        if value is not None:
+            print(str(value))
     except AttributeError:
         parser.error('no such property: {!r}'.format(args.property))