mscoree: Partial implement ICLRMetaHost RequestRuntimeLoadedNotification (try 2)

[Vincent Povirk]

 HRESULT CLRMetaHost_CreateInstance(REFIID riid, void **ppobj)
 {
+    GlobalCLRMetaHost.callback = NULL;
     return ICLRMetaHost_QueryInterface(&GlobalCLRMetaHost.ICLRMetaHost_iface,
riid, ppobj);
 }

I don't think we should be changing global state every time someone
creates an instance of this object. We can't assume it only happens
once.