monitor: Fix QMP ABI breakage around "id"

Commit 65207c5 accidentally dropped a line of code we need along with
a comment that became wrong then.  This made QMP reject "id":

    {"execute": "system_reset", "id": "1"}
    {"error": {"class": "GenericError", "desc": "QMP input object member 'id' is unexpected"}}

Put the lost line right back, so QMP again accepts and returns "id",
as promised by the ABI:

    {"execute": "system_reset", "id": "1"}
    {"return": {}, "id": "1"}

Reported-by: Fabio Fantoni <fabio.fantoni@m2r.biz>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Don Slutz <dslutz@verizon.com>
Tested-by: Fabio Fantoni <fabio.fantoni@m2r.biz>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Tested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1433753070-12632-2-git-send-email-armbru@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
