block: vhdx metadata table parsing

Parse the metadata table, and find the table entry fields for the:
    file parameters
    virtual disk size
    page 83 data
    logical sector size
    physical sector size
    parent locator

Still TODO: error handling if we don't find one of the required
entries.

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.