foreign-menu: Don't show empty foreign menu on secondary displays
    
    When using ovirt://, the foreign menu will only be shown in the primary
    window after getting notified about OvirtForeignMenu::files (ie when
    it managed to fetch some ISO files to show in the foreign menu).
    
    However, for secondary windows, the foreign menu will be added to the
    window even if there are no files to show. This commit makes sure we
    destroy the window foreign menu whenever it would be empty.