UefiCpuPkg/DxeMpLib: Place APs to suitable state on Legacy OS boot

Currently, DxeMpLib only places APs into specified c-state in Exit Boot Service
callback function for UEFI OS boot. We need to put APs into specified c-state
for legacy OS boot also.

https://bugzilla.tianocore.org/show_bug.cgi?id=210

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>

UefiCpuPkg/Library/MpInitLib/DxeMpInitLib.inf

@@ -58,6 +58,7 @@
 
 [Guids]
   gEfiEventExitBootServicesGuid                 ## CONSUMES  ## Event
+  gEfiEventLegacyBootGuid                       ## CONSUMES  ## Event
 
 [Pcd]
   gUefiCpuPkgTokenSpaceGuid.PcdCpuMaxLogicalProcessorNumber        ## CONSUMES