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>