block: add test for vhdx image created by Disk2VHD

This adds a test for VHDX images created by Microsoft's tool, Disk2VHD.

VHDX images created by this tool have 2 identical header sections, with
identical sequence numbers.  This makes sure we detect VHDX images with
identical headers, and do not flag them as corrupt.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
