qemu: add support for generating -audiodev arguments

The -audiodev argument is replacing the QEMU_AUDIO_DRV env variable (and
its relations).

Sadly we still have to use the SDL_AUDIODRIVER env variable because that
wasn't mapped into QAPI schema.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

tests/qemuxml2argvdata/x86_64-pc-headless.x86_64-latest.args

@@ -6,7 +6,6 @@ LOGNAME=test \
 XDG_DATA_HOME=/tmp/lib/domain--1-guest/.local/share \
 XDG_CACHE_HOME=/tmp/lib/domain--1-guest/.cache \
 XDG_CONFIG_HOME=/tmp/lib/domain--1-guest/.config \
-QEMU_AUDIO_DRV=none \
 /usr/bin/qemu-system-x86_64 \
 -name guest=guest,debug-threads=on \
 -S \
@@ -47,6 +46,7 @@ addr=0x2 \
 -chardev socket,id=charchannel0,fd=1729,server=on,wait=off \
 -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,\
 id=channel0,name=org.qemu.guest_agent.0 \
+-audiodev id=audio1,driver=none \
 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6 \
 -object rng-random,id=objrng0,filename=/dev/urandom \
 -device virtio-rng-pci,rng=objrng0,id=rng0,bus=pci.0,addr=0x7 \