audio: honor QEMU_AUDIO_TIMER_PERIOD instead of waking up every *nano* second

Now that we no longer have MIN_REARM_TIMER_NS a bug in the audio subsys has
clearly shown it self by trying to make a timer fire every nano second.

Note we have a similar problem in 1.6, 1.5 and older but there
MIN_REARM_TIMER_NS limits the wakeups caused by audio being active to
4000 times / second. This still causes a host cpu load of 50 % for simply
playing audio, where as with this patch git master is at 13%, so we should
backport this to 1.5 and 1.6 too.

Note this will not apply to 1.5 and 1.6 as is.

Cc: qemu-stable@nongnu.org
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
