block: for vhdx, check for all required metadata headers.

TODO: If file parameters has_parent is set, we also need to flag
      vhdx_parent_locator_header as required.

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.
block: split vhdx specification typedefs into vhdx header file

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.