virtio-blk: use blk_io_plug/unplug for Linux AIO batching

The raw-posix block driver implements Linux AIO batching so multiple
requests can be submitted with a single io_submit(2) system call.
Batching is currently only used by virtio-scsi and
virtio-blk-data-plane.

Enable batching for regular virtio-blk so the number of io_submit(2)
system calls is reduced for workloads with queue depth > 1.

In 4KB random read performance tests with queue depth 32, the CPU
utilization on the host is reduced by 9.4%.  The fio job is as follows:

  [global]
  bs=4k
  ioengine=libaio
  iodepth=32
  direct=1
  sync=0
  time_based=1
  runtime=30
  clocksource=gettimeofday
  ramp_time=5

  [job1]
  rw=randread
  filename=/dev/vdb
  size=4096M
  write_bw_log=fio
  write_iops_log=fio
  write_lat_log=fio
  log_avg_msec=1000

This benchmark was run on an raw image on LVM.  The disk was an SSD
drive and -drive cache=none,aio=native was used.

Tested-by: Pradeep Surisetty <psuriset@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>

hw/block/virtio-blk.c

@@ -600,13 +600,17 @@ static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue *vq)
         return;
     }
 
+    blk_io_plug(s->blk);
+
     while ((req = virtio_blk_get_request(s))) {
         virtio_blk_handle_request(req, &mrb);
     }
 
     if (mrb.num_reqs) {
         virtio_blk_submit_multireq(s->blk, &mrb);
     }
+
+    blk_io_unplug(s->blk);
 }
 
 static void virtio_blk_dma_restart_bh(void *opaque)