Fix "Change of linked base VM doesn't work with templates migrated from 2.0"

gns3server/controller/appliance.py

@@ -44,7 +44,9 @@ class Appliance:
         # Version of the gui before 2.1 use linked_base
         # and the server linked_clone
         if "linked_base" in self._data:
-            self._data["linked_clone"] = self._data.pop("linked_base")
+            linked_base = self._data.pop("linked_base")
+            if "linked_clone" not in self._data:
+                self._data["linked_clone"] = linked_base
         if data["node_type"] == "iou" and "image" in data:
             del self._data["image"]
         self._builtin = builtin