kern/52199: vioscsi kernel panic on a KVM hypervisor

[code%boerschig.net@localhost]

>Number:         52199
>Category:       kern
>Synopsis:       vioscsi kernel panic on a KVM hypervisor
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Apr 27 19:10:00 +0000 2017
>Originator:     MB
>Release:        7.1
>Organization:
>Environment:
>Description:
I installed NetBSD on a "virtual server" on an ISP's Linux/KVM-based hypervisor. 
The installation using a 'rescue environment' works fine.
However, he KVM in normal operation supports only virtio-scsi for disks.
After a couple of minutes the machine dies, I tried updating the kernel but that corrupted the kernel file and/or the filesystem.
Using a VNC-console I captured the following (transcript from screenshot):
[...lots of HBA errors ...]
sd0(vioscsi0:0:1:0): generic HBA error
sd0(vioscsi0:0:1:0): adapter resource shortage
sd0(vioscsi0:0:1:0): generic HBA error
sd0(vioscsi0:0:1:0): generic HBA error
fatal page fault in supervisor mode
trap type 6 code 0 rip ffffffff80974049 cs 8 rflags 10286 cr7 70 ilevel 6 rsp fffffe8002faedc8
curlwp 0xfffffe803fe12000 pid 0.4 lowest kstack 0xfffffe8002fac2c0
kernel: page fault trap, code=0
Stopped in pid 0.4 (system) at netbsd:vioscsi_vq_done+0x13c: mov	70(%r12),%eax
db{0}> bt
vioscsi_vq_done() at netbsd:vioscsi_vq_done+0x13c
virtio_vq_intr() at netbsd:virtio_vq_intr+0x70
virtio_intr() at netbsd:virtio_intr+0x38
intr_biglock_wrapper() at netbsd:intr_biglock_wrapper+0x19
Xintr_ioapic_level5() at netbsd:Xintr_ioapic_level5+0xf2
--- interrupt ---
Xspllower() at netbsd:Xspllower+0xe
DDB lost frame for netbsd:Xsoftintr+0x4f, trying 0xffffe8002faeff0
Xsoftintr() at netbsd:Xsoftintr+0x4f
--- interrupt ---
0:
db{0}>

>How-To-Repeat:
Run a virtual machine with NetBSD 7.1 on a Linux/KVM server with virtio-scsi enabled.
>Fix: