qmp: Don't reuse qmp visitor after grabbing output

The testsuite was the only client that attempted to reuse a
QmpOutputVisitor for a second visit after encountering an
error and/or calling qmp_output_get_qobject() on a first
visit.  The next patch is about to tighten the semantics to
be one-shot usage of the visitor, like all other visitors
(which will enable further simplifications down the road).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1462854006-24658-1-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>