Illumos 6358 - A faulted pool with only unavailable vdevs

6358 A faulted pool with only unavailable vdevs triggers assertion
failure in libzfs
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Andrew Stormont <andyjstormont@gmail.com>
Reviewed by: Serban Maduta <serban.maduta@gmail.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:

https://illumos.org/issues/6358
https://github.com/illumos/illumos-gate/commit/b289d04

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