zfs: image installation does not work

Created by: ollieparanoid

@locusf: https://gist.github.com/locusf/3a285ff1b68bc516c8db20e95bc28f4a

@MartijnBraam

that should be a fixed issue because there is a fallback with truncate implemented #28 (closed) I guess fallocate still creates an empty file so the fallback isn't used