Update both the developer and spec for the new QMP OOB (Out-Of-Band)
command.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-2-peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: grammar tweaks]
Signed-off-by: Eric Blake <eblake@redhat.com>