target/ppc: Restrict 'qapi-commands-machine.h' to system emulation

target/ppc: 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.

Move the QMP functions from cpu_init.c (which is always compiled)
to monitor.c (which is only compiled when system-emulation
is selected).  Rename monitor.c to arm-qmp-cmds.c.

Note ppc_cpu_class_by_name() is used by both file units, so we
expose its prototype in "cpu-qom.h".

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20230223155540.30370-5-philmd@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>