fix(getinstdisk): choose the driver group before the identifier

The scan wrote each disk into a file named after the identifier it
reported, wwn, path or neither, and read back the groups of one such
file only. Two disks that reported different identifiers therefore
never competed on their driver group: a disk without a WWN was dropped
as soon as another disk reported one, and when the last disk scanned
reported a WWN the readback opened the WWN files alone. A direct
attached boot disk that reports no WWN thus lost to a RAID volume that
reports one, which is the case the driver groups exist to decide.

Write every disk into the file of its driver group and keep the
identifier as the sort key inside that group, ranked so that a WWN
sorts ahead of a path and a path ahead of no identifier. The driver
group now decides first for every disk, the identifier still decides
between disks of one group, and no disk is dropped from the scan.