Error: Device 768 (vbd) could not be connected. Backend device not found.

["Jonathan M. McCune" <jonmccune@cmu.edu>]

Hello,

I am getting this error when I try to start an HVM guest on an AMD 
Sahara system.  The xen-unstable-hvm.hg revision 8724 works fine.  I 
tried to upgrade to xen-unstable.hg revision 9668.  I got 9668 to boot 
using 'yaird' to build the initrd for the -xen kernel. I can 
successfully start a paravirtualized guest, but I am now having trouble 
starting an HVM guest.  The error message is:

Error: Device 768 (vbd) could not be connected. Backend device not found.

I googled a bit and other people have seen this error, but no solutions
seem to be presented.

Each attempt to start the domain puts a line like the following in
/var/log/xen-hotplug.log (The '8' here is the domain ID):

xenstore-read: couldn't read path backend/vbd/8/768/node

Here is my config file:

kernel = "/usr/lib/xen/boot/hvmloader"
builder='hvm'
memory = 260
name = "Sarge"
disk = [ 'file:/sarge/disk.img,ioemu:hda,w' ]
vif = [ 'type=ioemu, mac=00:14:bf:52:df:5e, bridge=xenbr0' ]
device_model = '/usr/lib/xen/bin/qemu-dm'
memmap = '/usr/lib/xen/boot/mem-map.sxp'
boot='c'
sdl=1
vnc=0

I have made sure that xend and xenstored are running, but I'm not sure 
what else to check (it works on the older Xen!).  Any suggestions would 
be greatly appreciated.

Thanks,
-Jon