softmmu: add means to pass an exit code when requesting a shutdown

softmmu: add means to pass an exit code when requesting a shutdown

As of now, the exit code was either EXIT_FAILURE when a panic shutdown
was requested or EXIT_SUCCESS otherwise.
However, some hardware could want to pass more complex exit codes. Thus,
introduce a new shutdown request function allowing that.

Signed-off-by: Clément Chigot <chigot@adacore.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20231003071427.188697-2-chigot@adacore.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>