qmp: fix qmp_capabilities error regression

qmp: fix qmp_capabilities error regression

When someone sends a command before QMP handshake, the error used to be
like this:

 {"execute": "query-cpus"}
 {"error": {"class": "CommandNotFound", "desc":
            "Expecting capabilities negotiation with 'qmp_capabilities'"}}

While after cf869d5317 it becomes:

 {"execute": "query-cpus"}
 {"error": {"class": "CommandNotFound", "desc":
            "The command query-cpus has not been found"}}

Fix it back to the nicer one.

Fixes: cf869d5317 ("qmp: support out-of-band (oob) execution", 2018-03-19)
Reported-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180326063901.27425-2-peterx@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: commit message grammar tweaks]
Signed-off-by: Eric Blake <eblake@redhat.com>