Illumos 5147 - zpool list -v should show individual disk capacity

The 'zpool list -v' command displays lots of info but excludes the
capacity of each disk. This should be added.

Reviewed by: Christopher Siden christopher.siden@delphix.com
Reviewed by: Adam Leventhal adam.leventhal@delphix.com
Reviewed by: Matthew Ahrens matthew.ahrens@delphix.com

References:
  https://www.illumos.org/projects/illumos-gate//issues/5147
  https://reviews.csiden.org/r/89

Ported by: Turbo Fredriksson <turbo@bayour.com>