Revert to red label while restoring on missing labels

If we allow users to create custom labels and they backup such qubes,
restore operation will fail on new systems without those custom labels.
Fix this by reverting to red label and allow users to restore their
backup.

fixes: QubesOS/qubes-issues#9781

qubesadmin/backup/restore.py

@@ -2020,6 +2020,12 @@ def _restore_vms_metadata(self, restore_info):
                 # can't create vm, but need backup info
                 new_vm = self.backup_app.domains[vm_name]
             else:
+                # do not fail if VM had a custom label. revert to red label
+                if not vm.label in self.app.labels:
+                    self.log.warning(
+                        'Reverting to red label while restoring %s ' \
+                        'since %s label does not exist', vm.name, vm.label)
+                    vm.label = "red"
                 try:
                     # first only create VMs, later setting may require other VMs
                     # be already created

qubesadmin/tests/backup/backupcompatibility.py

@@ -695,7 +695,7 @@
         },
         'test-net': {
             'klass': 'AppVM',
-            'label': 'red',
+            'label': 'scarlet',
             'properties': {
                 'maxmem': '300',
                 'memory': '300',
@@ -749,8 +749,11 @@ def assertCorrectlyConverted(self, backup_app, expected_data):
                 expected_data['domains'][vm]['template'], vm)
             self.assertEqual(backup_app.domains[vm].klass,
                 expected_data['domains'][vm]['klass'], vm)
-            self.assertEqual(backup_app.domains[vm].label,
-                expected_data['domains'][vm]['label'], vm)
+            if expected_data['domains'][vm]['label'] == 'scarlet':
+                self.assertEqual(backup_app.domains[vm].label, 'red', vm)
+            else:
+                self.assertEqual(backup_app.domains[vm].label,
+                    expected_data['domains'][vm]['label'], vm)
             self.assertEqual(backup_app.domains[vm].backup_path,
                 expected_data['domains'][vm]['backup_path'], vm)
             self.assertEqual(backup_app.domains[vm].included_in_backup,
@@ -1487,6 +1490,9 @@ def setup_expected_calls(self, parsed_qubes_xml, templates_map=None):
             [orig_admin_vm_list] + \
             [orig_admin_vm_list + b''.join(extra_vm_list_lines)] * \
             len(extra_vm_list_lines)
+        self.app.expected_calls[('dom0', 'admin.label.List', None, None)] = (
+            b'0\0red\norange\nyellow\ngreen\ngray\nblue\npurple\nblack\n'
+        )
 
     def mock_appmenus(self, queue, vm, stream):
         queue.put((vm.name, 'appmenus', None, stream.read()))