hw/virtio/virtio-nsm: Respond with correct length

hw/virtio/virtio-nsm: Respond with correct length

When we return a response packet from NSM, we need to indicate its
length according to the content of the response. Prior to this patch, we
returned the length of the source buffer, which may confuse guest code
that relies on the response size.

Fix it by returning the response payload size instead.

Fixes: bb154e3e0cc715 ("device/virtio-nsm: Support for Nitro Secure Module device")
Reported-by: Vikrant Garg <vikrant1garg@gmail.com>
Signed-off-by: Alexander Graf <graf@amazon.com>
Message-Id: <20250213114541.67515-1-graf@amazon.com>
Reviewed-by: Dorjoy Chowdhury <dorjoychy111@gmail.com>
Fixes: bb154e3e0cc715 (&quot;device/virtio-nsm: Support for Nitro Secure Module device&quot;)<br>
Reported-by: Vikrant Garg <vikrant1garg@gmail.com>
Signed-off-by: Alexander Graf <graf@amazon.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Vikrant Garg <vikrant1garg@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 131fe64e63c88ec52c45a5946a478c0edeb31b78)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>