virtio-crypto: use the correct length for cipher operation

In some modes of cipher algorithms, the length of destination data
maybe larger then source data, such as ciphertext stealing (CTS).

For symmetric algorithms, the length of ciphertext is definitly
equal to the plaintext for each crypto operation. So we should
use the src_len instead of dst_len avoid to pass the incorrect
cryptographical results to the frontend driver.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-crypto.c

@@ -355,7 +355,7 @@ virtio_crypto_sym_input_data_helper(VirtIODevice *vdev,
         return;
     }
 
-    len = sym_op_info->dst_len;
+    len = sym_op_info->src_len;
     /* Save the cipher result */
     s = iov_from_buf(req->in_iov, req->in_num, 0, sym_op_info->dst, len);
     if (s != len) {