AudioUnitGetProperty would fail with kParamErr (-50) every time,
simply because size wasn't initialized.
Now it returns zero latency, but at least it doesn't fail.
https://bugzilla.gnome.org/show_bug.cgi?id=750868
Now that device selection has no sink/source-specific bits, we can have
generic device selection for this path. We do need to now track state
changes so we can look up the final device_id once the device is open,
though.
https://bugzilla.gnome.org/show_bug.cgi?id=740987
