port-pmax/54114: pmax on gxemul hangs with "UNIMPLEMENTED SCSI command"

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

>Number:         54114
>Category:       port-pmax
>Synopsis:       pmax on gxemul hangs with "UNIMPLEMENTED SCSI command"
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-pmax-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Apr 11 07:25:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date 2019.04.11.00.17.44
>Organization:

>Environment:
Architecture: mips
Machine: pmax
>Description:

Testing NetBSD-current/pmax under gxemul 0.6.1, the install RAMDISK
kernel kernel now hangs after printing the lines

  [   1.0000000] asc0 at tc0 slot 5 offset 0x0: NCR53C94, 25MHz, SCSI ID 7
  [   1.0000000] scsibus0 at asc0: 8 targets, 8 luns per target
  [   1.0039142] scsibus0: waiting 2 seconds for devices to settle...
  [ UNIMPLEMENTED SCSI command 0xa3, disk id=0 ]

The "UNIMPLEMENTED SCSI command" message comes from gxemul.  The
problem appeared some time between source dates 2019.03.04.05.37.08
and 2019.04.08.00.47.21.

Full log at:

  http://www.gson.org/netbsd/bugs/build/pmax/2019/2019.04.11.00.17.44/install.log

>How-To-Repeat:

>Fix: