Pass an iter to st_write_event_mapping, so the function can interate
different things depending on how we initialize the iter.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20210601132414.432430-5-kraxel@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>