Problem using blktap with xen-4.1 running Ubuntu Precise stock kernel with dom0 xen-4.1.

I get:

[    5.580106] XENBUS: Waiting for devices to initialise: 295s...290s.
...
[  300.580288] XENBUS: Timeout connecting to device: device/vbd/51713 (local state 3, remote state 1)

And some syslog lines:

May 17 13:07:30 localhost logger: /etc/xen/scripts/blktap: add XENBUS_PATH=backend/tap/10/51713
May 17 13:07:31 localhost logger: /etc/xen/scripts/blktap: Writing backend/tap/10/51713/hotplug-status connected to xenstore.

with tap:aio: disk lines. file:/ works.

disk = [
    'tap:aio:/data/root.img,xvda1,w',
]

Problem exists with lucid and precises domU kernels and both guests work in Ubuntu hardy dom0 Host 64bit 2.6.24-28-xen xen-3.3

 3.2.0-24-generic #37-Ubuntu SMP Wed Apr 25 08:43:22 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

Distributor ID: Ubuntu
Description:    Ubuntu 12.04 LTS
Release:        12.04
Codename:       precise