remove reflection and just use switches. 

 a little brute force, but more understandable.

Loops in zpool list, but eliminates a lot of other cruft.
```
[zsh|matt@servnerr-2]:~ 0 % sudo zfs list -p primary
NAME              USED          AVAIL   REFER  MOUNTPOINT
primary  3998631942464  3603057518272  273792  /primary
```

It would appear my installation does now! (Ubuntu Server 14.04LTS, libzfs2 0.6.3-2~trusty)

:+1: for just using bytes though.  If the caller wants to make them human readable, that does seem outside the scope of this package.