accel/tcg: Restrict 'qapi-commands-machine.h' to system emulation

accel/tcg: Restrict 'qapi-commands-machine.h' to system emulation

Since commit a0e61807a3 ("qapi: Remove QMP events and commands from
user-mode builds") we don't generate the "qapi-commands-machine.h"
header in a user-emulation-only build.

Rename 'hmp.c' as 'monitor.c' and move the QMP functions from
cpu-exec.c (which is always compiled) to monitor.c (which is only
compiled when system-emulation is selected).

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20221219170806.60580-4-philmd@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>