cpu: Introduce sanity checks when switching between CPUs

cpu: Introduce sanity checks when switching between CPUs

This patch introduces the following sanity checks when switching
between CPUs:

 * Check that the set of new and old CPUs do not overlap. Having an
   overlap between the set of new CPUs and the set of old CPUs is
   currently not supported. Doing such a switch used to result in the
   following assertion error:
     BaseCPU::takeOverFrom(BaseCPU*): \
       Assertion `!new_itb_port->isConnected()' failed.

 * Check that all new CPUs are in the switched out state.

 * Check that all old CPUs are in the switched in state.