device: start DHCPv6 when a prefix delegation is needed

If a prefix delegation is needed, currently NM restarts DHCPv6 on the device with default route, but only if DHCPv6 was already running.

Allow the device to start DHCPv6 for a PD even if it was running without DHCPv6.

See also: https://github.com/coreos/fedora-coreos-tracker/issues/888