hv: prepare for down/up APs dynamically.

- export start_cpus to start/online APs.
- Add stop_cpus to offline APs.
- Update cpu_dead to decrement running cpus number and do cleanup
  for AP down

Signed-off-by: Yin Fegnwei <fengwei.yin@intel.com>
Acked-by: Eddie Dong <Eddie.dong@intel.com>

hypervisor/include/arch/x86/cpu.h

@@ -246,6 +246,7 @@ bool is_vapic_intr_delivery_supported(void);
 bool is_vapic_virt_reg_supported(void);
 bool cpu_has_cap(uint32_t bit);
 void load_cpu_state_data(void);
+void start_cpus();
 
 /* Read control register */
 #define CPU_CR_READ(cr, result_ptr)                         \