Illumos 5039 - ztest should default to larger device sizes

5039 ztest should default to larger device sizes
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Max Grossman <max.grossman@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

References:

https://www.illumos.org/issues/5039
https://github.com/illumos/illumos-gate/commit/539eed8

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>