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.