Add support for the spice audio interface.  With this patch applied
audio can be forwarded over the network from/to the spice client.  Both
recording and playback is supported.

The driver is first in the driver list, but the can_be_default flag is
set only in case spice is active.  So if you have the spice protocol
enabled the spice audio driver is the default one, otherwise whatever
comes first after spice in the list.  Overriding the default using
QEMU_AUDIO_DRV works in any case.

[ v2: audio codestyle: add spaces before open parenthesis ]
[ v2: add const to silence array ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Cc: malc <av1474@comtv.ru>
Signed-off-by: malc <av1474@comtv.ru>