zpool: do guid-based comparison in is_vdev_cb()

is_vdev_cb() uses string comparison to find a matching vdev and
will fallback to comparing the guid via a string. These changes
drop the string comparison and compare the guids instead.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Rob Wing <rob.wing@klarasystems.com>
Co-authored-by: Rob Wing <rob.wing@klarasystems.com>
Sponsored-by: Seagate Technology
Submitted-by: Klara, Inc.
Closes #14311