Unregisters the specified event handler (callback function) that was previously registered with the PrlSrv_RegEventHandler function.
When using callback functionality with asynchronous functions, a callback function is first registered with Parallels Service. When no longer needed (client doesn't want to receive events anymore), the function must by unregistered using the PrlSrv_UnregEventHandler function. The values of the handler and userData parameters constitute a unique pair which will be used to identify the event handler that will be unregistered. Make sure that you supply the same values as the ones you used when you registered the handler. See PrlSrv_SubscribeToHostStatistics for the complete example illustrating event handlers usage.