S5 function dose not work on Waag and RTVM

[YuanXin-Intel]

Bug description
On ACRN version 3.3, s5 function cannot work on Waag and RTVM. When service VM request Waag and RTVM to shutdown through S5 vUART port, Waag and RTVM would not response.

Platform
The issue is reproduces on ADL.

Codebase
ACRN master and release 3.3

Scenario
Industry

To Reproduce
Steps to reproduce the behavior:

1. Enable S5 function according to https://projectacrn.github.io/latest/tutorials/enable_s5.html
2. Launch Waag or RTVM
3. Send shutdown request to Waag or RTVM
4. See error

Expected behavior
The Waag or RTVM should shutdown.