Rework disk backlog alerts

- change backlog meaning and thresholds
  backlog as concurrency level
  (how many disk requests are currently
   being served or are queued)
- filter physical devices only
  LVMs (dm-), physical volumes (vd-)
  misrepresent physical disk backlog

Change-Id: Idaf8f3f8f6e6938f73ad7ca88f12429475ec2194
Related-PROD: PROD-36296
1 file changed