OvmfPkg: improve SMM comms security with adaptive MemoryTypeInformation

OvmfPkg: improve SMM comms security with adaptive MemoryTypeInformation

* In the Intel whitepaper:

--v--
A Tour Beyond BIOS -- Secure SMM Communication

https://github.com/tianocore/tianocore.github.io/wiki/EDK-II-Security-White-Papers
https://github.com/tianocore-docs/Docs/raw/master/White_Papers/A_Tour_Beyond_BIOS_Secure_SMM_Communication.pdf
--^--

bullet#3 in section "Assumption and Recommendation", and bullet#4 in "Call
for action", recommend enabling the (adaptive) Memory Type Information
feature.

* In the Intel whitepaper:

--v--
A Tour Beyond BIOS -- Memory Map and Practices in UEFI BIOS

https://github.com/tianocore/tianocore.github.io/wiki/EDK-II-white-papers
https://github.com/tianocore-docs/Docs/raw/master/White_Papers/A_Tour_Beyond_BIOS_Memory_Map_And_Practices_in_UEFI_BIOS_V2.pdf
--^--

figure#6 describes the Memory Type Information feature in detail; namely
as a feedback loop between the Platform PEIM, the DXE IPL PEIM, the DXE
Core, and BDS.

Implement the missing PlatformPei functionality in OvmfPkg, for fulfilling
the Secure SMM Communication recommendation.

In the longer term, OVMF should install the WSMT ACPI table, and this
patch contributes to that.

Notes:

- the step in figure#6 where the UEFI variable is copied into the HOB is
  covered by the DXE IPL PEIM, in the DxeLoadCore() function,

- "PcdResetOnMemoryTypeInformationChange" must be reverted to the DEC
  default TRUE value, because both whitepapers indicate that BDS needs to
  reset the system if the Memory Type Information changes.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=386
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200310222739.26717-6-lersek@redhat.com>
Acked-by: Leif Lindholm <leif@nuviainc.com>