fe069d9d had aligned code and documentation while dropping the s from the
actual JSON output. Fix that.

This also fix test/qemu-iotest/081 since the missing s was causing a permutation.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>