Add support for 'tag-created-vm-with' feature

When a VM with 'tag-created-vm-with' feature set creates a VM (using
Admin API), that VM will get all the tags listed in the feature.
Multiple tags can be separated with spaces.

This will be useful to tag VMs created during paranoid mode backup
restore.

QubesOS/qubes-issues#5310

qubes/ext/admin.py

@@ -142,3 +142,12 @@ class AdminExtension(qubes.ext.Extension):
         if hasattr(self, 'policy_cache'):
             self.policy_cache.cleanup()
             del self.policy_cache
+
+    @qubes.ext.handler('domain-tag-add:created-by-*')
+    def on_tag_add(self, vm, event, tag, **kwargs):
+        '''Add extra tags based on creators 'tag-created-vm-with' feature'''
+        # pylint: disable=unused-argument,no-self-use
+        created_by = vm.app.domains[tag.partition('created-by-')[2]]
+        tag_with = created_by.features.get('tag-created-vm-with', '')
+        for tag_with_single in tag_with.split():
+            vm.tags.add(tag_with_single)