QAudioDeviceInfo::defaultOutputDevice() doesn't work as expected

QAudioDeviceInfo::defaultOutputDevice() and defaultInputDevice() don't actually return the default devices, but simply the first device in the list of available devices.

Several plugins have an implementation to return the actual system's default device, but that's never actually used by QAudioDeviceInfo.

QAudioSystemPlugin doesn't even have virtual functions to retrieve the default devices.