vdpa: Accessing CVQ header through its structure

We can access the CVQ header through `struct virtio_net_ctrl_hdr`,
instead of accessing it through a `uint8_t` pointer,
which improves the code's readability and maintainability.

Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
Message-Id: <cd522e06a4371e9d6b8a1c1a86f90a92401d56e8.1688743107.git.yin31149@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

net/vhost-vdpa.c

@@ -934,6 +934,7 @@ static int vhost_vdpa_net_handle_ctrl_avail(VhostShadowVirtqueue *svq,
 {
     VhostVDPAState *s = opaque;
     size_t in_len;
+    const struct virtio_net_ctrl_hdr *ctrl;
     virtio_net_ctrl_ack status = VIRTIO_NET_ERR;
     /* Out buffer sent to both the vdpa device and the device model */
     struct iovec out = {
@@ -949,7 +950,9 @@ static int vhost_vdpa_net_handle_ctrl_avail(VhostShadowVirtqueue *svq,
     out.iov_len = iov_to_buf(elem->out_sg, elem->out_num, 0,
                              s->cvq_cmd_out_buffer,
                              vhost_vdpa_net_cvq_cmd_len());
-    if (*(uint8_t *)s->cvq_cmd_out_buffer == VIRTIO_NET_CTRL_ANNOUNCE) {
+
+    ctrl = s->cvq_cmd_out_buffer;
+    if (ctrl->class == VIRTIO_NET_CTRL_ANNOUNCE) {
         /*
          * Guest announce capability is emulated by qemu, so don't forward to
          * the device.