block: fix vhdx file param structure field.

The bitfield in the file parameters metadata is a uin64_t, not uint32_t
block: vhdx, code movement for the header help macros
block: vhdx support for the file parameter metadata

This reads the file parameter metadata, including whether or not
the image has a parent, the virtual disk size, logical sector size,
physical sector size, and the block size.

Warning: this is a commit in my 'working' repo, and will likely
         by rebased and squashed into a new branch before submission
         to qemu-devel.