cpu: cpu_by_arch_id() helper

The helper can be used for CPU object lookup using the CPU's
arch-specific ID (the one returned by CPUClass::get_arch_id()).

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
[Yi Wang: Added documentation comments]
Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Signed-off-by: Yun Liu <liu.yunh@zte.com.cn>
[ehabkost: extracted cpu_by_arch_id() to a separate patch]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>