MFC r348586: MFV r348585: 9683 Allow bypassing devid in vdev_disk_open()

illumos/illumos-gate@6fe4f3002ca33af5ab7123ed78d81899dadf2fbb

Reviewed by: Sara Hartse <sara.hartse@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Pavel Zakharov <pavel.zakharov@delphix.com>

This is irrelevant to FreeBSD, just to reduce divergence.