MigTD can use serial console to print log messages including warn/errors/fatal

Currently, debugging MigTD is quite hard. We had to put a debug TDVMCALL in order to see why MigTD is behaving in a certain way. This issue is making a suggestion to instead use VGA screen and/or serial console to print log messages including warn/error/fatal ones.

The debug messages can be part of the cfg and warn/error/fatals ones can always go to the VGA screen and/or serial port.

MigTD can also use a shared buffer and then VMM can implement a TDVMCALL to get the PA of the string + length but this option requires changes in the VMM so instead a serial port maybe a better option. VGA screen buffer is small so maybe serial port is the best option if it feasible to implement in MigTD. The shared buffer is common whether we log it to serialport or VGA screen or over the TDVMCALL.

No files or tests are named. Start by tracing MigTD's existing debug TDVMCALL logging path, then compare the proposed serial console, VGA screen, and shared-buffer approaches, including the cfg distinction for debug versus warn/error/fatal messages. Done means the chosen logging path is defined and those message levels are visible without requiring a debug TDVMCALL.

Written by the indexing model from the issue text.