block: vhdx header for the QEMU support of VHDX images

This is based on Microsoft's VHDX specification:
    "VHDX Format Specification v0.95", published 4/12/2012
    https://www.microsoft.com/en-us/download/details.aspx?id=29681



These structures define the various header, metadata, and other
block structures defined in the VHDX specification.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>