target-m68k: Detect attempt to instantiate non-CPU type in cpu_init()

Consolidate model checking into a new m68k_cpu_class_by_name().

If the name matches an existing type, also check whether that type is
(a sub-type of) TYPE_M68K_CPU.

This fixes, e.g., -cpu ide-hd asserting.

Signed-off-by: Andreas Färber <afaerber@suse.de>
