cpus: Introduce CPUClass::list_cpus() callback

cpus: Introduce CPUClass::list_cpus() callback

Some targets define cpu_list to a method listing their
CPUs on stdout. In order to make list_cpus() generic,
introduce the CPUClass::list_cpus() callback.
When no callback is registered, list_cpus() defaults
to the cpu_list definition.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Message-Id: <20250324185837.46506-2-philmd@linaro.org>