The detach-disk command does not work for tap device, since
xenStoreDomainGetDiskID is always searching for vbd device from
XenStore. c.f. This is a the related patch:
http://git.et.redhat.com/?p=libvirt.git;a=commit;h=d27506d3a36e12b977f6a8...
So, I changed the searching path to both vbd and tap.
Regards,
Saori Fukuta