CoreAudio: Allow more flexability when specifying SampleRates

The available sample rates for a given device are the nominal sample
rates reported by the device.  It is possible to use other sample rates
and CoreAudio will take care of the conversion.  So what we will do is
report what rates are available for a device, but not explicitly require
those sample rates to have a valid format.

Task-number: QTBUG-36265
Change-Id: Idbbdeacbb6bc1fe434bcd8dec519ad70d4ccd545
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
AVFoundation: Improve config test

This should prevent the avfoundation plugin from being built on system
that can not support using it (ex. 32bit builds on OS X 10.7)

Task-number: QTBUG-36239
Change-Id: I9f861b7580b0e015ddbc1231440d677bf6eee1ec
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>