UefiCpuPkg/DxeMpLib: Rename MpInitExitBootServicesCallback()

UefiCpuPkg/DxeMpLib: Rename MpInitExitBootServicesCallback()

Rename MpInitExitBootServicesCallback() to MpInitChangeApLoopCallback() because
it will not only be invoked on Exit Boot Service Event, but also will be invoked
on Legacy Ready To Boot Event.

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>