Update vdev state for spare vdev

zfsd fetches new pool configuration through ZFS_IOC_POOL_STATS but
it does not get updated nvlist configuration for spare vdev since
the configuration is read by spa_spares->sav_config. In this commit,
updating the vdev state for spare vdev that is consumed by zfsd on
spare disk hotplug.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #14653