add two cpu model objects for physical and virtual

add two cpu model objects for physical and virtual

We delete surf_cpu_model, which was only one cpu model object on the
system. Now, we add surf_cpu_model_pm and surf_cpu_model_vm for the
physical and virtual machine layers, respectively.

FIXME: cpu_ti will not work correctly because using global variables. We
have to update cpu_ti to support multiple cpu model objects.