virtio-net: Fix num_buffers for version 1

The specification says the device MUST set num_buffers to 1 if
VIRTIO_NET_F_MRG_RXBUF has not been negotiated.

Fixes: df91055db5c9 ("virtio-net: enable virtio 1.0")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20250108-buffers-v1-1-a0c85ff31aeb@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/net/virtio-net.c

@@ -1999,6 +1999,8 @@ static ssize_t virtio_net_receive_rcu(NetClientState *nc, const uint8_t *buf,
                                     offsetof(typeof(hdr),
                                              virtio_net.hdr.num_buffers),
                                     sizeof(hdr.virtio_net.hdr.num_buffers));
+            } else {
+                hdr.virtio_net.hdr.num_buffers = cpu_to_le16(1);
             }
 
             guest_offset = n->has_vnet_hdr ?