vhost: implement vhost_force_iommu method

use the vhost_force_iommu callback to force enable feature bit VIRTIO_F_IOMMU_PLATFORM

Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20200701145538.22333-12-lulu@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>

hw/virtio/vhost.c

@@ -810,6 +810,11 @@ static int vhost_dev_set_features(struct vhost_dev *dev,
     if (!vhost_dev_has_iommu(dev)) {
         features &= ~(0x1ULL << VIRTIO_F_IOMMU_PLATFORM);
     }
+    if (dev->vhost_ops->vhost_force_iommu) {
+        if (dev->vhost_ops->vhost_force_iommu(dev) == true) {
+            features |= 0x1ULL << VIRTIO_F_IOMMU_PLATFORM;
+       }
+    }
     r = dev->vhost_ops->vhost_set_features(dev, features);
     if (r < 0) {
         VHOST_OPS_DEBUG("vhost_set_features failed");