hw/arm/vexpress: Check for CPU types in machine_run_board_init()

hw/arm/vexpress: Check for CPU types in machine_run_board_init()

Leverage the common code introduced in commit c9cf636d48 ("machine:
Add a valid_cpu_types property") to check for the single valid CPU
type. Remove the now unused MachineClass::default_cpu_type field.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20240129151828.59544-9-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>