target-arm: Detect attempt to instantiate non-CPU type in cpu_init()

Consolidate model checking into a new arm_cpu_class_by_name().

If the name matches an existing type, also check whether that type is
actually (a sub-type of) TYPE_ARM_CPU.

This fixes, e.g., -cpu tmp105 asserting.

Cc: qemu-stable <qemu-stable@nongnu.org>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
