hw/core: Get default_cpu_type calling machine_class_default_cpu_type()

hw/core: Get default_cpu_type calling machine_class_default_cpu_type()

Since commit 62b4a227a33 the default cpu type can come from the
valid_cpu_types[] array. Call the machine_class_default_cpu_type()
instead of accessing MachineClass::default_cpu_type field.

Cc: qemu-stable@nongnu.org
Fixes: 62b4a227a33 ("hw/core: Add machine_class_default_cpu_type()")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Message-Id: <20250422084114.39499-1-philmd@linaro.org>
(cherry picked from commit d5f241834be1b323ea697a469ff0f1335a1823fe)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>