Illumos 4966 - zpool list iterator does not update output

4966 zpool list iterator does not update output
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Garrett D'Amore <garrett@damore.org>

References:

https://github.com/illumos/illumos-gate/commit/cd67d23
https://www.illumos.org/issues/4966

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3566