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

Consolidate model checking into a new openrisc_cpu_class_by_name().

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

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

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