replay: Use new QAPI cloning

Rather than rolling our own clone via an expensive conversion
in and back out of QObject, use the new clone visitor.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1465490926-28625-16-git-send-email-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
