zfs: allow to mount root from a pool not in zpool.cache

[Andreas Nilsson]

>
> Hostid is not used for root pool, but I'm aware current way of setting
> up ZFS this way is far from being intuitive and requires some
> understanding of how it work internally. It shouldn't be that way and
> I'm glad it will change.
>
> I'm glad to see those plans coming to fruition! I'll try to find the time
to test the patches during the weekend. My ultimate goal is beeing able to
boot from a zfs snapshot.

Best regards
Andreas