On Windows IID's are represented as GUID by value, instead of nsID
by reference on non-Windows platforms.

Patch the vbox_CAPI_v2_2.h header to deal with this difference.

Rewrite vboxIID abstraction that deals with the different IID
representations. Add support for the GUID representation. Also unify
the four context dependent free functions for vboxIIDs

  vboxIIDUnalloc, vboxIIDFree, vboxIIDUtf8Free, vboxIIDUtf16Free

into vboxIIDUnalloc that is now safe to be called (even multiple
times) on a vboxIID independent of the source and context of the
vboxIID.

The new vboxIID is designed to be used as a stack allocated variable.
It has a value member that represents the actual IID value.