virtio-crypto: fix virtio_queue_set_notification() race

We must check for new virtqueue buffers after re-enabling notifications.
This prevents the race condition where the guest added buffers just
after we stopped popping the virtqueue but before we re-enabled
notifications.

I think the virtio-crypto code was based on virtio-net but this crucial
detail was missed.  virtio-net does not have the race condition because
it processes the virtqueue one more time after re-enabling
notifications.

Cc: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>