Fixes Hyper-V issue with VHD file format

The VHD format cookie signature needs to be checked in the footer
instead of the header as fixed format VHDs don't have a copy of the
footer at the beginning of the file.

Fixes bug: #1233853

Change-Id: Ibe44f0d895cde2edbc03dcd6ecebac24b0936660
Merge "Fixes Hyper-V issue with VHD file format"