vbox: Make CreateHardDisk support all vbox versions
    
    The CreateHardDiskMedium only support create HardDisk for medium
    type, and it only works when vbox version >= 3.1. This patch make
    the function workable with all vbox versions and rename it as
    CreateHardDisk.
    
    In vbox 2.2 and 3.0 this function will create a IHardDisk object.
    In vbox later than 3.0, this function will create a IMedium object.