virtio-pci: Check if is_avq is NULL

[bug]
In the virtio_pci_common.c function vp_del_vqs, vp_dev->is_avq is involved
to determine whether it is admin virtqueue, but this function vp_dev->is_avq
 may be empty. For installations, virtio_pci_legacy does not assign a value
 to vp_dev->is_avq.

[fix]
Check whether it is vp_dev->is_avq before use.

[test]
Test with virsh Attach device
Before this patch, the following command would crash the guest system

After applying the patch, everything seems to be working fine.
Signed-off-by: Li Zhang <zhanglikernel@gmail.com>
Message-Id: <1710566754-3532-1-git-send-email-zhanglikernel@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_pci_common.c

@@ -236,7 +236,7 @@ void vp_del_vqs(struct virtio_device *vdev)
 	int i;
 
 	list_for_each_entry_safe(vq, n, &vdev->vqs, list) {
-		if (vp_dev->is_avq(vdev, vq->index))
+		if (vp_dev->is_avq && vp_dev->is_avq(vdev, vq->index))
 			continue;
 
 		if (vp_dev->per_vq_vectors) {