tools: fix handling single optional VM name

qubesadmin/tools/__init__.py

@@ -163,11 +163,19 @@ class VmNameAction(QubesAction):
             if hasattr(namespace, 'exclude') and namespace.exclude:
                 parser.error('--exclude can only be used with --all')
 
-            for vm_name in getattr(namespace, self.dest):
-                try:
-                    namespace.domains += [app.domains[vm_name]]
-                except KeyError:
-                    parser.error('no such domain: {!r}'.format(vm_name))
+            if self.nargs == argparse.OPTIONAL:
+                vm_name = getattr(namespace, self.dest, None)
+                if vm_name is not None:
+                    try:
+                        namespace.domains += [app.domains[vm_name]]
+                    except KeyError:
+                        parser.error('no such domain: {!r}'.format(vm_name))
+            else:
+                for vm_name in getattr(namespace, self.dest):
+                    try:
+                        namespace.domains += [app.domains[vm_name]]
+                    except KeyError:
+                        parser.error('no such domain: {!r}'.format(vm_name))
 
 
 class RunningVmNameAction(VmNameAction):