linux-user/main: Use list_cpus() instead of cpu_list()

This way we can get rid of the if'deffery and the XXX comment
here (it's repeated in the list_cpus() function anyway).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230424122126.236586-1-thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>