Manual: add info about QEMU_AUDIO_DRV
Manual: add info about QEMU_AUDIO_DRV

hramrach on IRC had an issue with audio not working. He was using
suse and his qemu defaulted to using the pulseaudio audio driver instead
of spice.  Explicitly setting the audio driver to spice using the
QEMU_AUDIO_DRV environment variable solved the problem. Add this tip to
the manual for others having the same issue in the future.

fgouget/spice,fgouget/spice,fgouget/spice,fgouget/spice