virtio: call backend-specific queue cleanup on vdev stop

This was omitted in the original implementation and the
`del_queue` callback wasn't called anywhere. For virtio-user
we open some eventfds on queue creation and until now they
were never being closed.

Change-Id: Iee1ced1e17a59d5cb13449538c115678a1c1a328
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/416999
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>

lib/virtio/virtio.c

@@ -211,6 +211,7 @@ virtio_free_queues(struct virtio_dev *dev)
 			continue;
 		}
 
+		virtio_dev_backend_ops(dev)->del_queue(dev, vq);
 		spdk_dma_free(vq->vq_ring_virt_mem);
 
 		rte_free(vq);