tests/unit/test-smp-parse: Restore MachineClass fields after modifying

tests/unit/test-smp-parse: Restore MachineClass fields after modifying

There is a single MachineClass object, registered with
type_register_static(&smp_machine_info). Since the same
object is used multiple times (an MachineState object
is instantiated in both test_generic and test_with_dies),
we should restore its internal state after modifying for
the test purpose.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Yanan Wang <wangyanan55@huawei.com>
Tested-by: Yanan Wang <wangyanan55@huawei.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211115145900.2531865-2-philmd@redhat.com>