kunit: tool: Allow overriding the shutdown mode from qemu config

Not all platforms support machine reboot.
If it a proper reboot is not supported the machine will hang.
Allow the QEMU configuration to override the necessary shutdown mode for
the specific system under test.

Link: https://lore.kernel.org/r/20241111-kunit-loongarch-v2-2-7676eb5f2da3@linutronix.de


Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>