qapi: change QmpOutputVisitor to QSLIST

This saves a little memory compared to the doubly-linked QTAILQ.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1467906798-5312-2-git-send-email-pbonzini@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Comment tweaked to avoid long line]
Signed-off-by: Markus Armbruster <armbru@redhat.com>