block: initial VHDX driver support framework - supports open and probe

This is the initial block driver framework for VHDX image support
(i.e. Hyper-V image file formats), that supports opening VHDX files, and
parsing the headers.

This commit does not yet enable:
    - reading
    - writing
    - updating the header
    - differencing files (images with parents)
    - log replay / dirty logs (only clean 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

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