virDomainActualNetDefContentsFormat: Format class_id more frequently
    
    After a360912179 the formatting of virDomainActualNetDefPtr was
    changed a bit. However, during the function rewrite, iface's class_id
    is not formatted as frequently as it could be. In fact, after rewrite
    it's formatted only for iface of type VIR_DOMAIN_NET_TYPE_DIRECT where
    it makes no sense and is unused. While where needed (_TYPE_NETWORK) is
    not formatted at all. This makes the daemon forget it upon daemon
    restart resulting in bad behaviour.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>