cpu: Make checker CPUs inherit from CheckerCPU in the Python hierarchy

cpu: Make checker CPUs inherit from CheckerCPU in the Python hierarchy

Checker CPUs currently don't inherit from the CheckerCPU in the Python
object hierarchy. This has two consequences:
 * It makes CPU model discovery from the Python world somewhat
   complicated as there is no way of testing if a CPU is a checker.
 * Parameters are duplicated in the checker configuration
   specification.

This changeset makes all checker CPUs inherit from the base checker
CPU class.