vbox: Rewrite vboxStoragePoolNumOfVolumes

vbox: Rewrite vboxStoragePoolNumOfVolumes

We use typedef IMedium IHardDisk to make IHardDisk hierachy from
IMedium (Actually it did on vbox 2.2 and 3.0's C++ API).
So when calling
    VBOX_MEDIUM_FUNC_ARG*(IHardDisk, func, args)
we can directly replace it to
    gVBoxAPI.UIMedium.func(IHardDisk, args)

When dealing with this two types, we get some rules from it's
hierachy relationship.

When using IHardDisk and IMedium as input, we can't transfer a
IMedium to IHardDisk. Like:
    gVBoxAPI.UIHardDisk.func(IHardDisk *hardDisk, args)
    Here, we can't put a *IMedium as a argument.

When using IHardDisk and IMedium as output, we can't transfer a
IHardDisk to IMedium. Like:
    gVBoxAPI.UIMachine.GetMedium(IMedium **out)
    Here, we can't put a **IHardDisk as a argument. If this case
    do happen, we either change the API to GetHardDisk or write a
    new one.