udpate examples/riscv/src/main.rs, add net send/recv test

examples/riscv/src/main.rs

@@ -137,11 +137,10 @@ fn virtio_input<T: Transport>(transport: T) {
 }
 
 fn virtio_net<T: Transport>(transport: T) {
-    let mut _net = VirtIONet::<HalImpl, T>::new(transport).expect("failed to create net driver");
-    // TODO: add tests for CI
-    //let mut buf = [0u8; 0x100];
-    //let len = net.recv(&mut buf).expect("failed to recv");
-    //info!("recv: {:?}", &buf[..len]);
-    //net.send(&buf[..len]).expect("failed to send");
+    let mut net = VirtIONet::<HalImpl, T>::new(transport).expect("failed to create net driver");
+    let mut buf = [0u8; 0x100];
+    let len = net.recv(&mut buf).expect("failed to recv");
+    info!("recv: {:?}", &buf[..len]);
+    net.send(&buf[..len]).expect("failed to send");
     info!("virtio-net test finished");
 }