MFV 295046: 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>
Author: Dan Vatca <dan.vatca@gmail.com>

illumos/illumos-gate@b289d045e084af53efcc025255af8242e41f28fa