block: vhdx - account for identical header sections

The VHDX spec v1.00 declares that "a header is current if it is the only
valid header or if it is valid and its SequenceNumber field is greater
than the other header’s SequenceNumber field. The parser must only use
data from the current header. If there is no current header, then the
VHDX file is corrupt."

However, the Disk2VHD tool from Microsoft creates a VHDX image file that
has 2 identical headers, including matching checksums and matching
sequence numbers.  Likely, as a shortcut the tool is just writing the
header twice, for the active and inactive headers, during the image
creation.  Technically, this should be considered a corrupt VHDX file
(at least per the 1.00 spec, and that is how we currently treat it).

But in order to accomodate images created with Disk2VHD, we can safely
create an exception for this case.  If we find identical sequence
numbers, then we check the VHDXHeader-sized chunks of each 64KB header
sections (we won't rely just on the crc32c to indicate the headers are
the same).  If they are identical, then we go ahead and use the first
one.

Reported-by: Nerijus Baliūnas <nerijus@users.sourceforge.net>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
