netfilter_set_position and netfilter_set_netdev_id overwrote their
respective fields with g_strdup without freeing the previous value,
leaking memory on each property write.
Fixes: fdccce4596 ("init/cleanup of netfilter object")
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>