RFC for zfs tests. 

This is a work in process. 

This patchset adds support, documentation, and tests for installing NixOS to a ZFS partition. Booting off of ZFS is probably possible with a sufficient of grub-fu, but currently only separate boot/root is tested. 

There is also a new test for running the zfs unit tests. 

Dodgy bits:
- I had to bump up the amount of memory for the VMs from 384MiB to 768MiB. Don't know a way around this, but maybe we could isolate it to this test?
- I changed the qemu cache policy for the virtual drives from 'writeback' to 'writethrough', as ztest was failing
- I added 'zfs' to supported filesystems on the ISO. I'm not sure if this makes the ISO distributable (because of the CDDL). This is probably fixable by installing the zfs software/kernel modules before creating the filesystems, but I'm not sure the best way to do that.