hv: keylocker: host keylocker iwkey context switch

Different vCPU may have different IWKeys. Hypervisor need do the iwkey
context switch.

This patch introduce a load_iwkey() function to do that. Switches the
host iwkey when the switch_in vCPU satisfies:
  1) keylocker feature enabled
  2) Different from the current loaded one.

Two opportunities to do the load_iwkey():
  1) Guest enables CR4.KL bit.
  2) vCPU thread context switch.

load_iwkey() costs ~600 cycles when do the load IWKey action.

Tracked-On: #5695
Signed-off-by: Shuo A Liu <shuo.a.liu@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

hypervisor/include/arch/x86/per_cpu.h

@@ -59,6 +59,7 @@ struct per_cpu_region {
 #endif
 	uint64_t shutdown_vm_bitmap;
 	uint64_t tsc_suspend;
+	struct acrn_vcpu *whose_iwkey;
 	/*
 	 * We maintain a per-pCPU array of vCPUs. vCPUs of a VM won't
 	 * share same pCPU. So the maximum possible # of vCPUs that can