QMP: Only handle converted commands

Looks like I dropped this check when addressing the 'query-'
commands request.

QMP should only handle converted commands, obviously.

Reported-by: Markus Armbruster <armbru@redhat.com>

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
