VM.js setting the `inet6` if the manifest contains `dhcp6` for the interface

Because `mdata-fetch` delete all `/etc/dhcp.${iface}` by booting a zone it's also required to setup the `inet6` if `dhcp6`.

* Reference: https://github.com/arekinath/smartos-live/blob/eait/src/vm/node_modules/VM.js#L2384
* mdata-fetch requires also to set `inet6` for the dhcp interface config