target-unicore32: Detect attempt to instantiate non-CPU type in cpu_init()

Consolidate model checking into a new uc32_cpu_class_by_name().

If the name matches an existing type, also check whether that type is
actually (a sub-type of) TYPE_UNICORE32_CPU.

This fixes, e.g., -cpu puv3_dma asserting.

Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>
