CoreAudio: Make it possible to set volume on of QAudioOutput

QAudioOutput::setVolume stopped working for CoreAudio when it was ported
to live in it's own plugin.  This was because it was not possible to set
the volume of QAudioOutput in iOS.  Now the functionality has been
restored and added for iOS as well.  For OS X we use the old method of
setting the volume property of the AudioUnit.  On iOS it is not possible
to set the volume on a per AudioUnit basis, so we now manually modify
the buffer contents (the same we do for QAudioInput already).

Task-number: QTBUG-36756
Change-Id: I42b5892fe5534217043fa55e7b5b9a4ce824050d
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>