All vbox objects are child objects from the nsISupports in vbox's
C++ API version. Since the CAPI is generated from the C++ API, I
kept their relationship here, by the definitations below:

typedef struct nsISupports nsISupports;
typedef nsISupports IVirtualBox;
typedef nsISupports ISession;
and so on...

So, when calling the API from nsISupports, we don't need to do
typecasting, and things work still work well.