Expose zpool guids through kstats (#68)

[anodos325]

There are times when end-users may wish to have
a fast and convenient method to get zpool guid
without having to use libzfs. This commit
exposes the zpool guid via kstats in similar
manner to the zpool state.

Reviewed-by: Alexander Motin mav@FreeBSD.org
Reviewed-by: Brian Behlendorf behlendorf1@llnl.gov
Signed-off-by: Andrew Walker awalker@ixsystems.com
Closes openzfs#13466

Motivation and Context

Description

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.