virtio: hide legacy features from modern guests

NOTIFY_ON_EMPTY, ANY_LAYOUT and BAD are only valid on the legacy
interface.

Hide them from modern guests.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-pci.c

@@ -1095,7 +1095,8 @@ static uint64_t virtio_pci_common_read(void *opaque, hwaddr addr,
         break;
     case VIRTIO_PCI_COMMON_DF:
         if (proxy->dfselect <= 1) {
-            val = vdev->host_features >> (32 * proxy->dfselect);
+            val = (vdev->host_features & ~VIRTIO_LEGACY_FEATURES) >>
+                (32 * proxy->dfselect);
         }
         break;
     case VIRTIO_PCI_COMMON_GFSELECT:

include/hw/virtio/virtio.h

@@ -25,6 +25,10 @@
 /* A guest should never accept this.  It implies negotiation is broken. */
 #define VIRTIO_F_BAD_FEATURE		30
 
+#define VIRTIO_LEGACY_FEATURES ((0x1ULL << VIRTIO_F_BAD_FEATURE) | \
+                                (0x1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) | \
+                                (0x1ULL << VIRTIO_F_ANY_LAYOUT))
+
 struct VirtQueue;
 
 static inline hwaddr vring_align(hwaddr addr,