tcg: Introduce tcg_max_ctxs

Finish the divorce of tcg/ from hw/, and do not take
the max cpu value from MachineState; just remember what
we were passed in tcg_init.

Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>