Add information about hostid to guide installing rootfs on zfs #916

I went with instructions on how to install void rootfs on zfs with separate partitions for /boot and /boot/efi.

https://docs.voidlinux.org/installation/guides/zfs.html

After I installed successfully void I get following situation.

1. I would run command xbps-reconfigure -fa
2. Reboot.
3. System does not boot because zfs pool can not be imported because there is hostid mismatch in zfs.
4. Boot into livecd hrmpf.
5. Import and export pool: zpool import -N -R /mnt mypool , zpool export mypool.
6. Reboot
7. System boots successfully.

I fixed this doing:

1. I created file /etc/hostid with zgenhostid -f $(hostid)
2. I added install_items+=" /etc/hostid " to /etc/dracut.conf.d/zol.conf
3. Now system boots after doing xbps-reconfigure -fa

I suggest to add info about generating and setting hostid to guide to avoid problems later.