tcg: Pass max_threads not max_cpus to tcg_init

tcg: Pass max_threads not max_cpus to tcg_init

In effect, hoist the check for mttcg from tcg_n_regions()
to tcg_init_machine().

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>