qemu-iotests: Add data pattern in version3 VMDK sample image in 059

It's possible that we diverge from the specification with our
implementation.  Having a reference image in the test cases may detect
such problems when we introduce a bug that can read what it creates, but
can't handle a real VMDK.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>