Expose zpool guids through kstats

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 #13466

include/sys/spa.h

@@ -898,6 +898,7 @@ typedef struct spa_stats {
 	spa_history_kstat_t	tx_assign_histogram;
 	spa_history_list_t	mmp_history;
 	spa_history_kstat_t	state;		/* pool state */
+	spa_history_kstat_t	guid;		/* pool guid */
 	spa_history_kstat_t	iostats;
 } spa_stats_t;