Now VM operations workig!

Now VM operations workig!

- Fix the resource deallocation on exit, i.e., delete VM-related objects
  from host_lib without calling the callbacks.
- Implement vm_{suspend/resume, save/restore, shutdown}
- Call SIMIX_simcall_answer() at the end of some SIMIX_functions, so
  that avoid the bug that these functions never return to the userland.
- Delete the use of some simcalls at the SIMIX layer.
- Implement  vm_migrate. It updates the net_elm and PM of the VM.
- Add vm_ws_execute() to add action cost to the vcpu's dummy execution
  action.
- Delete the code around GUESTOS_NOISE. we need to add something like a
  task periodically.