docs: new design document multi-thread-tcg.txt

This documents the current design for upgrading TCG emulation to take
advantage of modern CPUs by running a thread-per-CPU. The document goes
through the various areas of the code affected by such a change and
proposes design requirements for each part of the solution.

The text marked with (Current solution[s]) to document what the current
approaches being used are.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>