Be sure to plumb also inet6 interfaces for dhcp6 support

* it is required to plumb the interface for dhcp6 support also
* this patch only works for primary dhcp6 interface because it read
  inet informatiom from /etc/dhcp.${iface}
VM.js setting the `inet6` if the manifest contains `dhcp6` for the interface

* 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