tests/qmp-test: Demonstrate QMP errors jumping the queue

tests/qmp-test: Demonstrate QMP errors jumping the queue

When OOB is enabled, out-of-band commands are executed right away,
everything else is queued.  This lets out-of-band commands "jump the
queue".

However, certain errors are always reported right away, and therefore
can jump the queue even when the erroneous input does not request
out-of-band execution.  These errors are pretty unlikely to occur in
production, but it's wrong all the same.  Mark FIXME.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180703085358.13941-17-armbru@redhat.com>