virDomainActualNetDefContentsFormat: Format class_id more frequently

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>