tcg: Convert TCGState::mttcg_enabled to TriState

tcg: Convert TCGState::mttcg_enabled to TriState

Use the OnOffAuto type as 3-state.

Since the TCGState instance is zero-initialized, the
mttcg_enabled is initialzed as AUTO (ON_OFF_AUTO_AUTO).

In tcg_init_machine(), if mttcg_enabled is still AUTO,
set a default value (effectively inlining the
default_mttcg_enabled() method content).

In the tcg_get_thread() getter, consider AUTO / OFF states
as "single", otherwise ON is "multi".

Reviewed-by: Anton Johansson <anjo@rev.ng>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>