vhost-vdpa: correctly return err in vhost_vdpa_set_backend_cap()

We should return error code instead of zero, otherwise there's no way
for the caller to detect the failure.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20210903091031.47303-3-jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost-vdpa.c

@@ -443,13 +443,13 @@ static int vhost_vdpa_set_backend_cap(struct vhost_dev *dev)
     int r;
 
     if (vhost_vdpa_call(dev, VHOST_GET_BACKEND_FEATURES, &features)) {
-        return 0;
+        return -EFAULT;
     }
 
     features &= f;
     r = vhost_vdpa_call(dev, VHOST_SET_BACKEND_FEATURES, &features);
     if (r) {
-        return 0;
+        return -EFAULT;
     }
 
     dev->backend_cap = features;