storage/file: add is_outdated() that always returns False

Fixes QubesOS/qubes-issues#6398

qubes/storage/file.py

@@ -219,6 +219,9 @@ class FileVolume(qubes.storage.Volume):
         if self.snap_on_start or self.save_on_stop:
             _remove_if_exists(self.path_cow)
 
+    def is_outdated(self):
+        return False  # avoid spamming the log with NotImplementedError
+
     def is_dirty(self):
         if self.save_on_stop:
             with suppress(FileNotFoundError), open(self.path_cow, 'rb') as cow: