Keep forever .gns3 backup before version upgrade

gns3server/controller/topology.py

@@ -98,7 +98,7 @@ def load_topology(path):
     if "revision" not in topo or topo["revision"] < GNS3_FILE_FORMAT_REVISION:
         # If it's an old GNS3 file we need to convert it
         # first we backup the file
-        shutil.copy(path, path + ".backup")
+        shutil.copy(path, path + ".backup{}".format(topo.get("revision", 0)))
         topo = _convert_1_3_later(topo, path)
         with open(path, "w+", encoding="utf-8") as f:
             json.dump(topo, f)