hw/misc/sifive_test.c: replace exit calls with proper shutdown

hw/misc/sifive_test.c: replace exit calls with proper shutdown

This replaces the exit calls by shutdown requests, ensuring a proper
cleanup of Qemu. Otherwise, some connections like gdb could be broken
before its final packet ("Wxx") is being sent. This part, being done
inside qemu_cleanup function, can be reached only when the main loop
exits after a shutdown request.

Signed-off-by: Clément Chigot <chigot@adacore.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20231003071427.188697-4-chigot@adacore.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>