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
block: add read-only support to VHDX image format.

This adds in read-only support to the VHDX image format.  This supports
reads for fixed-size, and dynamic sized VHDX images.

Differencing files are still unsupported.

The image must be opened without BDRV_O_RDWR set, because we do not
yet update the headers.  I.e., pass 'readonly=on' in the drive image
options from the QEMU commandline.
block: add header update capability for VHDX images.

This adds the ability to update the headers in a VHDX image, including
generating a new MS-compatible GUID, and checksum.
block: add write support for VHDX images

This adds in write support for VHDX images.  This supports writing to
both dynamic and fixed VHDX images.  Differencing images are still
unsupported.