Revise try_receive_message_by_uart to read one char from uart one time.
With this implementation each char can be checked. This can be used to
address the following 2 problems:
1) nosie data: it is found that there is noise data in the uart from
guest VM when guest startup.
2) split multiple commands
Tracked-On: #8111
Signed-off-by: Jian Jun Chen <jian.jun.chen@intel.com>
73ffa7d6d2