hw/core/machine: Split out the smp parsing code

We are going to introduce an unit test for the parser smp_parse()
in hw/core/machine.c, but now machine.c is only built in softmmu.

In order to solve the build dependency on the smp parsing code and
avoid building unrelated stuff for the unit tests, move the tested
code from machine.c into a separate file, i.e., machine-smp.c and
build it in common field.

Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211026034659.22040-2-wangyanan55@huawei.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>