A number of entries that can be present in the spa config shouldn't be saved
to disk so add a method to ensure this is case. Without this if the last
caller to vdev_config_generate requested stats then we can end up in the
cache file.
Also only skip a none writable pool in the cache file generation if its
active. This prevents unavailable pools incorrectly getting removed from
cache file.
Tested by: delphij
MFC after: 2 weeks
Sponsored by: Multiplay
3d07512cea