In case of external plugins xfce_panel_plugin_position_menu() returns incorrect old coordinates if plugin was moved to the another place on the panel.

To reproduce:
1. Start panel with all plugins as external (force-all-external true).
2. Add "Directory Menu" plugin.
3. Move plugin to the another place on the panel.
4. Click on the plugin, the menu will be opened at the old plugin's position.

This behavior can be reproduced with another external plugins too,
see https://bugzilla.xfce.org/show_bug.cgi?id=7317.