storage/reflink: no-op import_volume() if not save_on_stop

Instead of raising a NotImplementedError, just return self like 'file'
and lvm_thin. This is needed when Storage.clone() is modified in another
commit* to no longer swallow exceptions.

* "storage: factor out _wait_and_reraise(); fix clone/create"

qubes/storage/reflink.py

@@ -276,7 +276,8 @@ class ReflinkVolume(qubes.storage.Volume):
         return self
 
     def import_volume(self, src_volume):
-        self._require_save_on_stop('import_volume')
+        if not self.save_on_stop:
+            return self
         try:
             _copy_file(src_volume.export(), self._path_import)
         except: