vbox: Make FindMedium support old vbox versions
    
    In old version, function FindMedium in UIVirtualBox doesn't work
    for vbox2.2 and 3.0. We assume it will not be used when vbox in
    these versions.
    
    But when rewriting vboxStorageVolLookupByPath, we found it was
    compatibe to use FindMedium to get a IHardDisk object, even in
    vbox old versions. To achieve this, first make FindMedium call
    FindHardDisk when VBOX_API_VERSION < 4000000.
    Then change the argument type **IMedium to **IHardDisk. (As the
    rules in heriachy, we can't transfer a IHardDisk to match
    IMedium in output)
    
    In vbox 2.2 and 3.0, the caller must be aware that they will get
    a IHardDisk object in return.