audio: propagate Error * out of audio_init

Starting from audio_driver_init, propagate errors via Error ** so that
audio_init_audiodevs can simply pass &error_fatal, and AUD_register_card
can signal faiure.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
[Reworked the audio/audio.c parts, while keeping Martin's hw/ changes. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>