The next commit needs to access compat policy from the generic visitor
core.  Move it there from qobject input and output visitor.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20211025042405.3762351-4-armbru@redhat.com>