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

target/loongarch: 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.

Extract the QMP functions from cpu.c (which is always compiled)
to the new 'loongarch-qmp-cmds.c' unit (which is only compiled
when system emulation is selected).

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230223155540.30370-4-philmd@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>