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

(cherry-picked from commit fdc5a57d03b9a53981dc0e1623091980570456eb)

Conflicts:

	nova/tests/virt/hyperv/test_vhdutils.py

Change-Id: Ibe44f0d895cde2edbc03dcd6ecebac24b0936660
Merge "Fixes Hyper-V issue with VHD file format" into stable/havana