OvmfPkg: include UefiCpuPkg/CpuMpPei

OvmfPkg: include UefiCpuPkg/CpuMpPei

In the next patch we're going to put EFI_PEI_MP_SERVICES_PPI to use.

CpuMpPei uses the following PCDs from gUefiCpuPkgTokenSpaceGuid, beyond
those already used by CpuDxe:

- PcdCpuMicrocodePatchAddress and PcdCpuMicrocodePatchRegionSize: these
  control whether CpuMpPei performs microcode update. If the region size
  is zero, then the microcode update is skipped. UefiCpuPkg.dec sets the
  region size to zero by default, which is appropriate for OVMF.

- PcdCpuApLoopMode and PcdCpuApTargetCstate: the former controls how
  CpuMpPei puts the APs to sleep: 1 -- HLT, 2 -- MWAIT, 3 -- busy wait
  (with PAUSE). The latter PCD is only relevant if the former PCD is 2
  (MWAIT). In order to be consistent with SeaBIOS and with CpuDxe itself,
  we choose HLT. That's the default set by UefiCpuPkg.dec.

Furthermore, although CpuMpPei could consume SecPeiCpuExceptionHandlerLib
technically, it is supposed to consume PeiCpuExceptionHandlerLib. See:

- http://thread.gmane.org/gmane.comp.bios.edk2.devel/12703

- git commit a81abf161666 ("UefiCpuPkg/ExceptionLib: Import
  PeiCpuExceptionHandlerLib module"), part of the series linked above.

Jeff recommended to resolve CpuExceptionHandlerLib to
PeiCpuExceptionHandlerLib for all PEIMs:

- http://thread.gmane.org/gmane.comp.bios.edk2.devel/14471/focus=14477

Since at the moment we have no resolution in place that would cover this
for PEIMs (from either [LibraryClasses] or [LibraryClasses.common.PEIM]),
it's easy to do.

Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>