domcreate_launch_dm: unable to add disk devices

[smj%ma.sdf.org@localhost]

Recently did a upgrade from:

NetBSD 5.1
xentools33-3.3.2nb14
xenkernel33-3.3.2nb6

Which worked quite well.

to

NetBSD 7
xentools42-4.2.5nb15 
xenkernel42-4.2.5nb10

(I've tried 45-4.5.2 as well). 

The behaviour I'm seeing is DOMUs can start, and run .. but after a while,
usually within 24 hours any running DOMUs will start to hang and starting
a new DOMU will produce output like this:

libxl: error: libxl_device.c:875:device_backend_callback: unable to connect device with path /local/domain/0/backend/vbd/87/51712
libxl: error: libxl_device.c:875:device_backend_callback: unable to connect device with path /local/domain/0/backend/vbd/87/51728
libxl: error: libxl_create.c:951:domcreate_launch_dm: unable to add disk devices
libxl: error: libxl_dm.c:1269:libxl__destroy_device_model: could not find device-model's pid for dom 87
libxl: error: libxl.c:1419:libxl__destroy_domid: libxl__destroy_device_model failed for 87
libxl: error: libxl_device.c:875:device_backend_callback: unable to disconnect device with path /local/domain/0/backend/vbd/87/51712
libxl: error: libxl_device.c:875:device_backend_callback: unable to disconnect device with path /local/domain/0/backend/vbd/87/51728
libxl: error: libxl.c:1455:devices_destroy_cb: libxl__devices_destroy failed for 87

Rebooting fixes the problem.  vnconfig -l doesn't show anything strange, only
the allocated virtual disks for the running domus.  I have trace on, but
no real details other than the output above.

If going back to xen33 would fix it, then I have no problem doing that.