This copies the code implementing the policy from qapi/qmp-dispatch.c
to qapi/qobject-input-visitor.c.  Tolerable, but if we acquire more
copies, we should look into factoring them out.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Peter Krempa <pkrempa@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Message-Id: <20211025042405.3762351-5-armbru@redhat.com>