linux-aio: Fix laio resource leak

when hotplug virtio-scsi disks using laio, the aio_nr will
increase in laio_init() by io_setup(), we can see the number by
  # cat /proc/sys/fs/aio-nr
  128
if the aio_nr attach the maxnum, which found from
  # cat /proc/sys/fs/aio-max-nr
  65536
the hotplug process will fail because of aio context leak.

Fix it by io_destroy in laio_cleanup().

Reported-by: daifulai <daifulai@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>