Add audio device support

Add audio device support

Until now audio devices had to be added manually with the `args` option
in the VMs config file. This adds a new config option to define an audio
device. It is called `audio0` with the extra `0` to be prepared should
we ever implement support for more than one audio device.

Supported devices are:

* ich9-intel-hda: Intel HD Audio Controller, emulates ICH9
* intel-hda: Intel HD Audio Controller, emulates ICH6
* AC97: useful for older OSs like Win XP

If any of the `intel-hda`s is selected two additional devices will be
created: hda-micro and hda-duplex. The `*intel-hda` controllers need at
least one of them as they emulate microphones and speakers (hda-micro)
or line in and out devices (hda-duplex).

Having both is deemed best practice to avoid problems if some software
needs one of the other kind of output/input ports.

I also cleaned up some old, commented out, code regarding audio devices.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>