The only thing I can think of is that there at least used to be problems with sparse files. I use dd to create image files, with if=/dev/zero and fill in the file entirely. I don't know exactly what qemu-img-xen does. I wonder if you are having a filesystem locking bug problem, rather than a xen problem.
Attachment:
pgp50pd1z5y50.pgp
Description: PGP signature