port-xen/50659: Xen4.5 crash with vnds

[sebastian%xenoserver.net@localhost]

>Number:         50659
>Category:       port-xen
>Synopsis:       Xen4.5 crash with vnds
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    port-xen-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Jan 15 20:20:00 +0000 2016
>Originator:     Sebastian
>Release:        Netbsd 7.0
>Organization:
>Environment:
NetBSD xxx 7.0.0_PATCH NetBSD 7.0.0_PATCH (XEN3_DOM0.201601140120Z) amd64
>Description:
I have problems running a DOMU with VND backend disks. During DOMU startup i get errors from the backend:

root@xxx:~$ xl create -c /etc/xen/xx 
Parsing config from /etc/xen/xx
vnconfig: vnconfig: /dev/vnd0d: opendisk/dev/vnd0d: opendisk: : Device busy
Device busy
vnconfig: /dev/vnd1d: opendisk: Device busy
libxl: error: libxl_aoutils.c:464:async_exec_timeout: killing execution of /usr/pkg/etc/xen/scripts/block /local/domain/0/backend/vbd/1/51712 because of timeout
libxl: error: libxl_aoutils.c:464:async_exec_timeout: killing execution of /usr/pkg/etc/xen/scripts/block /local/domain/0/backend/vbd/1/51728 because of timeout
libxl: error: libxl_exec.c:129:libxl_report_child_exitstatus: /usr/pkg/etc/xen/scripts/block /local/domain/0/backend/vbd/1/51728 [-1] died due to fatal signal Killed
libxl: error: libxl_exec.c:129:libxl_report_child_exitstatus: /usr/pkg/etc/xen/scripts/block /local/domain/0/backend/vbd/1/51712 [-1] died due to fatal signal Killed
libxl: error: libxl_aoutils.c:464:async_exec_timeout: killing execution of /usr/pkg/etc/xen/scripts/block /local/domain/0/backend/vbd/1/51760 because of timeout
libxl: error: libxl_exec.c:129:libxl_report_child_exitstatus: /usr/pkg/etc/xen/scripts/block /local/domain/0/backend/vbd/1/51760 [-1] died due to fatal signal Killed
libxl: error: libxl_create.c:1153:domcreate_launch_dm: unable to add disk devices
xenstore-read: couldn't read path /local/domain/0/backend/vbd/1/51728/vnd
xenstore-read: couldn't read path /local/domain/0/backend/vbd/1/51760/vnd
libxl: info: libxl.c:1691:devices_destroy_cb: forked pid 2374 for destroy of domain 1

After the unsuccesful startup the xl comand hangs. A DOMU with less than 4 DOMUs starts.

Then after a reboot command the DOM0 crashes.
>How-To-Repeat:
1. Start DOMU with more than 4 VNDs backend disk
2. Try to reboot DOM0
>Fix: