fix thread migration code (i.e., sched_setaffinity())

- moved migration code into idle() process and updated schedule() to detect
  when a thread has moved to another CPU in order to avoid doing housekeeping
  on behalf of the original one
- start CPU head from core 0
- keeps track of nested interrupts

arch/x86/kernel/cpu.c

@@ -612,9 +612,8 @@ void handle_interrupt(int vector, struct x86_user_context *regs)
 	struct ihk_mc_interrupt_handler *h;
 	struct cpu_local_var *v = get_this_cpu_local_var();
 
-	v->in_interrupt = 1;
-
 	lapic_ack();
+	++v->in_interrupt;
 
 	dkprintf("CPU[%d] got interrupt, vector: %d, RIP: 0x%lX\n", 
 	         ihk_mc_get_processor_id(), vector, regs->gpr.rip);
@@ -684,7 +683,7 @@ void handle_interrupt(int vector, struct x86_user_context *regs)
 	check_signal(0, regs, 0);
 	check_need_resched();
 
-	v->in_interrupt = 0;
+	--v->in_interrupt;
 }
 
 void gpe_handler(struct x86_user_context *regs)