According to systemd-nspawn(1), --network-bridge implies --network-veth,
and --port option is supported only when private networking is enabled.
Fixes#52417.
Previously, setting "privateNetwork = true" without specifying host and
local addresses would create unconfigured interfaces: ve-$INSTANCE on the host
and eth0 inside the container.
These changes is rebased part of the original PR #3021.