hw/virtio/vring/event_idx: fix the vring_avail_event error

The event idx in virtio is an effective way to reduce the number of
interrupts and exits of the guest. When the guest puts an request
into the virtio ring, it doesn't exit immediately to inform the
backend. Instead, the guest checks the "avail" event idx to determine
the notification.

In virtqueue_pop, when a request is poped, the current avail event
idx should be set to the number of vq->last_avail_idx.

Signed-off-by: Bin Wu <wu.wubin@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
