qemu-iotests: fix pattern for write test

The write pattern value 0axb is invalid and evaluates to 0,
so the read check (which uses a correct value of 0xab) will fail.

This failure will only be detected with a separate patch for
qemu-io. Without it, qemu-io cannot interpret hex values
and always uses a pattern value of 0.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
