sandbox: pep8/fix vdir

buildstream/sandbox/sandbox.py

@@ -147,7 +147,7 @@ class Sandbox():
     def set_virtual_directory(self, vdir):
         """ Sets virtual directory. Useful after remote execution
         has rewritten the working directory. """
-        self.__vdir = vdir
+        self._vdir = vdir
 
     def get_virtual_toplevel_directory(self):
         """Fetches the sandbox's toplevel directory