usb: add audio device model

This brings a usb audio device to qemu.  Output only, fixed at
16bit stereo @ 480000 Hz.  Based on a patch from
H. Peter Anvin <hpa@linux.intel.com>

Usage: add '-device usb-audio' to your qemu command line.

Works sorta ok on a idle machine.  Known issues:

 * Is *very* sensitive to latencies.
 * Burns quite some CPU due to usb polling.

In short:  It brings the qemu usb emulation to its limits.  Enjoy!

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
