toos: fix handling default command (qvm-device, qvm-volume, ...)

When command isn't specified, command variable is set to None. Lets
handle this situation gracefuly.

qubesadmin/tools/__init__.py

@@ -389,6 +389,8 @@ class QubesArgumentParser(argparse.ArgumentParser):
                     argparse._SubParsersAction):  # pylint: disable=no-member
                 assert hasattr(namespace, 'command')
                 command = namespace.command
+                if command is None:
+                    continue
                 subparser = action._name_parser_map[command]
                 for subaction in subparser._actions:
                     if issubclass(subaction.__class__, QubesAction):

qubesadmin/tools/qvm_device.py

@@ -201,6 +201,10 @@ def get_parser(device_class=None):
     else:
         parser.add_argument('devclass', metavar='DEVICE_CLASS', action='store',
             help="Device class to manage ('pci', 'usb', etc)")
+
+    # default action
+    parser.set_defaults(func=list_devices)
+
     sub_parsers = parser.add_subparsers(
         title='commands',
         description="For more information see qvm-device command -h",

qubesadmin/tools/qvm_volume.py

@@ -192,6 +192,7 @@ def init_extend_parser(sub_parsers):
     extend_parser.add_argument('size', help='New size in bytes')
     extend_parser.set_defaults(func=extend_volumes)
 
+
 def get_parser():
     '''Create :py:class:`argparse.ArgumentParser` suitable for
     :program:`qvm-block`.
@@ -207,6 +208,8 @@ def get_parser():
     init_extend_parser(sub_parsers)
     init_list_parser(sub_parsers)
     init_revert_parser(sub_parsers)
+    # default action
+    parser.set_defaults(func=list_volumes)
 
     return parser