tests/unit/test-smp-parse: No need to explicitly zero MachineClass members

tests/unit/test-smp-parse: No need to explicitly zero MachineClass members

The default value of the MachineClass members is 0, which
means we don't have to explicitly zero them. Also the value
of "mc->smp_props.prefer_sockets" will be taken care of by
smp_parse_test(), we don't necessarily need the statement
in machine_base_class_init() either.

Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211228092221.21068-6-wangyanan55@huawei.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>