Let CPUClass::class_by_name() handlers to return abstract classes,
and filter them once in the public cpu_class_by_name() method.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230908112235.75914-3-philmd@linaro.org>