Unload should not delete VMs, just close them.

gns3server/modules/base_manager.py

@@ -97,9 +97,9 @@ class BaseManager:
     @asyncio.coroutine
     def unload(self):
 
-        for uuid in list(self._vms.keys()):
+        for uuid in self._vms.keys():
             try:
-                yield from self.delete_vm(uuid)
+                yield from self.close_vm(uuid)
             except Exception as e:
                 log.warn("Could not delete VM {}: {}".format(uuid, e))
                 continue