AC97 can allocate ~500MB of host RAM

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1603779

While working with qtest test cases generated via fuzzing with QEMU 2.5.0,
I discovered some odd behavior for the AC97 virtual device with qemu-
system-i386. If AC97_MIC_ADC_RATE is set to the value of 1, the QEMU
process allocates over 500MB of additional host RAM. You probably would
not normally notice this on a modern PC, except that I was using a
"ulimit" command to restrict the maximum amount of virtual memory allowed
for the QEMU process, so the process would crash with a SIGTRAP (signal 5)
on the failed memory allocation.

My minimized qtest code to reproduce the issue is:

static void test_crash(void)
{
  uint64_t barsize;
  dev = get_device();

  dev_base[0] = qpci_iomap(dev, 0, &barsize);
  dev_base[1] = qpci_iomap(dev, 1, &barsize);
  qpci_device_enable(dev);
  qpci_io_writew(dev, dev_base[0]+0x32, 0x00000001);
}

I ran a "ulimit -sv 650000" command and then launched the tests/ac97-test
binary with this crash test case included in it. I can then see the QEMU
process crash on an allocation of 722538464 bytes. I can gradually
increase the ulimit memory limit to ~1200000 and then no longer see the
issue, hence my estimate of 500 MB of RAM allocated by the device.