block/qapi: Emit tray_open only if there is a tray Signed-off-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Alberto Garcia <berto@igalia.com> Message-id: 1454096953-31773-5-git-send-email-mreitz@redhat.com