audio: Never send migration section

The audio migration vmstate is empty, and always has been; we can't
just remove it though because an old qemu might send it us.
Changes with -audiodev now mean it's sometimes created when it didn't
used to be, and can confuse migration to old qemu.

Change it so that vmstate_audio is never sent; if it's received it
should still be accepted, and old qemu's shouldn't be too upset if it's
missing.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Tested-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210809170956.78536-1-dgilbert@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>