audio: Convert use of atoi to qemu_strtoi

If qemu_strtoi indicates an error, return the default value.

Signed-off-by: Nia Alarie <nia.alarie@gmail.com>
Message-Id: <20180316144047.30904-1-nia.alarie@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>