qmp qemu-ga: Revert change that accidentally made qemu-ga accept "id"

Commit cf869d53 "qmp: support out-of-band (oob) execution" changed
how we check "id":

    Note that in the patch I exported qmp_dispatch_check_obj() to be
    used to check the request earlier, and at the same time allowed
    "id" field to be there since actually we always allow that.

The part after "and" is ill-advised: it makes qemu-ga accept and
ignore "id".  Revert.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180703085358.13941-10-armbru@redhat.com>