With xen-3.0.4-6fc7, and a configuration like this:

disk = [ "tap:aio:/tmp/test.img,xvda,w" ]

I'm seeing this:

----
[2007-03-02 10:08:09 xend.XendDomainInfo 2900] DEBUG (XendDomainInfo:94)
XendDomainInfo.create(['vm', ['name', 'test'], ['memory', 512], ['maxmem', 512],
['on_poweroff', 'destroy'], ['on_reboot', 'restart'], ['on_crash', 'restart'],
['vcpus', 1], ['on_xend_start', 'ignore'], ['on_xend_stop', 'ignore'], ['uuid',
'9e68f448-1a66-87f3-c327-69ecc7187692'], ['bootloader', '/usr/bin/pygrub'],
['image', ['linux', ['kernel', '/var/run/xend/boot/boot_kernel.9ZEo8m'],
['ramdisk', '/var/run/xend/boot/boot_ramdisk.i29Kd6'], ['args', 'ro
root=/dev/VolGroup00/LogVol00']]], ['device', ['tap', ['uname',
'tap:aio:/tmp/test.img'], ['dev', 'xvda'], ['mode', 'w']]], ['device', ['vif',
['bridge', 'xenbr0'], ['mac', '00:16:3e:6c:05:00']]]])
[2007-03-02 10:08:09 xend.XendConfig 2900] WARNING (XendConfig:607) Unconverted
key: cpus
[2007-03-02 10:08:09 xend.XendDomainInfo 2900] DEBUG (XendDomainInfo:1284)
XendDomainInfo.constructDomain
[2007-03-02 10:08:09 xend.XendDomainInfo 2900] DEBUG (XendDomainInfo:1330)
XendDomainInfo.initDomain: 1 256
[2007-03-02 10:08:09 xend.XendDomainInfo 2900] INFO (XendDomainInfo:1572)
Mounting /tmp/test.img on /dev/xvdp.
[2007-03-02 10:08:09 xend.XendDomainInfo 2900] DEBUG (XendDomainInfo:2035)
create_vbd_with_vdi: {'device': '/dev/xvdp', 'image': '/tmp/test.img', 'mode': 'RO'}
[2007-03-02 10:08:09 xend 2900] DEBUG (DevController:115) DevController: writing
{'backend-id': '0', 'virtual-device': '51952', 'device-type': 'disk', 'state':
'1', 'backend': '/local/domain/0/backend/tap/0/51952'} to
local/domain/0/device/vbd/51952
.
[2007-03-02 10:08:09 xend 2900] DEBUG (DevController:117) DevController: writing
{'domain': 'Domain-0', 'frontend': '/local/domain/0/device/vbd/51952', 'uuid':
'cf6bf81e-bbb4-ec20-1e8a-9ce7d540fcdd', 'dev': '/dev/xvdp', 'state': '1',
'params': 'qcow:/tmp/test.img', 'mode': 'r', 'online': '1', 'frontend-id': '0',
'type': 'tap'} to /local/domain/0/backend/tap/0/51952.
[2007-03-02 10:08:09 xend 2900] DEBUG (DevController:153) Waiting for 51952.
[2007-03-02 10:08:09 xend 2900] DEBUG (DevController:522) hotplugStatusCallback
/local/domain/0/backend/tap/0/51952/hotplug-status.
[2007-03-02 10:08:09 xend 2900] DEBUG (DevController:522) hotplugStatusCallback
/local/domain/0/backend/tap/0/51952/hotplug-status.
[2007-03-02 10:08:09 xend 2900] DEBUG (DevController:536) hotplugStatusCallback 1.
[2007-03-02 10:08:09 xend 2900] ERROR (XendBootloader:39) Disk isn't accessible
[2007-03-02 10:08:09 xend.XendDomainInfo 2900] INFO (XendDomainInfo:1590)
Unmounting /dev/xvdp from /dev/xvdp.
----

Don't you just love when things go wrong with Xen? "Disk isn't accessible"?
Geez, thanks.

So, it turns out that the problem is this:

[2007-03-02 10:08:09 xend 2900] DEBUG (DevController:117) DevController: writing
{'domain': 'Domain-0', ..., 'dev': '/dev/xvdp', ..., 'params':
'qcow:/tmp/test.img', ..., 'type': 'tap'} to /local/domain/0/backend/tap/0/51952.

This is a raw image, not a QCOW image, so it's failing to mount the image.

However, we're only trying to mount the image for pygrub, since pygrub can't
handle e.g. QCOW files directly. But it can handle raw images just fine.

So, attaching a patch to not attempt to mount the image if it's raw, which fixes
the bug for me.

(It doesn't fix the case of e.g. using pygrub with a vmdk image, but *shrug*)