kern/52614: qemu virtual CD-ROM report read errors since recent wdc changes

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         52614
>Category:       kern
>Synopsis:       qemu virtual CD-ROM reports read errors since recent wdc changes
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Oct 12 19:50:00 +0000 2017
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2017.10.07.20.02.07
>Organization:

>Environment:
System: NetBSD
Architecture: i386
Machine: i386
>Description:

The testbed on babylon5.netbsd.org has done more than 16,000
i386 installs in qemu over the last six years without printing
the string "cd0a: error reading" to the console even once.

This changed on source date 2017.10.07.20.02.07, which happens
to be immediately after a bunch of wdc related commits.  Since
then, the string with "cd0a: error reading" has been printed many
times:

  babylon5.netbsd.org$ zgrep -c 'cd0a: error reading' ./2017.10*/install.log.gz
  [44 lines showing zero matches omitted]
  ./2017.10.07.20.02.07/install.log.gz:5
  ./2017.10.07.20.32.20/install.log.gz:2
  ./2017.10.07.21.53.16/install.log.gz:1
  ./2017.10.08.00.45.25/install.log.gz:3
  ./2017.10.08.01.05.13/install.log.gz:8
  ./2017.10.08.03.39.50/install.log.gz:2
  ./2017.10.08.08.29.57/install.log.gz:5
  ./2017.10.08.09.10.11/install.log.gz:4
  ./2017.10.08.14.03.46/install.log.gz:0
  ./2017.10.08.15.00.40/install.log.gz:0
  ./2017.10.08.15.29.33/install.log.gz:2
  ./2017.10.08.18.46.10/install.log.gz:0
  ./2017.10.08.20.44.19/install.log.gz:1
  ./2017.10.08.21.18.14/install.log.gz:3
  ./2017.10.08.21.33.38/install.log.gz:1
  ./2017.10.09.05.24.26/install.log.gz:2
  ./2017.10.09.10.31.50/install.log.gz:0
  ./2017.10.09.12.07.03/install.log.gz:0
  ./2017.10.09.14.28.01/install.log.gz:6
  ./2017.10.09.17.49.28/install.log.gz:2
  ./2017.10.09.23.42.40/install.log.gz:2
  ./2017.10.10.03.11.01/install.log.gz:1
  ./2017.10.10.05.35.15/install.log.gz:3
  ./2017.10.10.09.29.14/install.log.gz:2
  ./2017.10.10.11.52.51/install.log.gz:3
  ./2017.10.10.13.47.27/install.log.gz:4
  ./2017.10.10.16.04.59/install.log.gz:2
  ./2017.10.10.16.44.24/install.log.gz:0
  ./2017.10.10.17.20.42/install.log.gz:1
  ./2017.10.10.19.31.57/install.log.gz:3
  ./2017.10.10.21.37.49/install.log.gz:1
  ./2017.10.11.00.17.03/install.log.gz:1
  ./2017.10.11.06.49.03/install.log.gz:9
  ./2017.10.11.08.29.17/install.log.gz:7
  ./2017.10.11.10.53.25/install.log.gz:1
  ./2017.10.11.12.27.49/install.log.gz:4
  ./2017.10.11.17.08.32/install.log.gz:0
  ./2017.10.12.09.53.55/install.log.gz:0

>How-To-Repeat:

Install NetBSD-current/i386 in qemu using a virtual CD-ROM.
Observe the console output.

>Fix: