block: vhdx - update PAYLOAD_BLOCK_UNMAPPED value to match 1.00 spec

The 0.95 VHDX spec defined PAYLOAD_BLOCK_UNMAPPED to be 5.  The 1.00
VHDX spec redefines PAYLOAD_BLOCK_UNMAPPED to be 3 instead.

The original value of 5 is now an undefined state in the spec, but it
should be safe to treat it the same and return zeros for data read.
This way, we can maintain compatibility with any images out in the wild
that may have been created in accordance to the 0.95 spec.

Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 8a4d2da73a8dbc04cde62bea782fc09ff84b1cf1.1418018421.git.jcody@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>