Read the last 8 bytes of fd
    and interpret it as an int.

def snapshot_peek_number( fd, off ):
    """
    Read the last 8 bytes of fd
    and interpret it as an int.
    """
    # read number of 8 bytes 
    fd.seek( off - 8, os.SEEK_SET )
    value_hex = fd.read(8)
    if len(value_hex) != 8:
        return None
    try:
        value = int(value_hex, 16)
    except ValueError:
        return None

    return value