OvmfPkg/PlatformPei: Reserve SEV-ES work area if S3 is supported

OvmfPkg/PlatformPei: Reserve SEV-ES work area if S3 is supported

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198

Protect the SEV-ES work area memory used by an SEV-ES guest.

Regarding the lifecycle of the SEV-ES memory area:
  PcdSevEsWorkArea

(a) when and how it is initialized after first boot of the VM

  If SEV-ES is enabled, the SEV-ES area is initialized during
  the SEC phase [OvmfPkg/ResetVector/Ia32/PageTables64.asm].

(b) how it is protected from memory allocations during DXE

  If SEV-ES is enabled, then InitializeRamRegions()
  [OvmfPkg/PlatformPei/MemDetect.c] protects the ranges with either
  an AcpiNVS (S3 enabled) or BootServicesData (S3 disabled) memory
  allocation HOB, in PEI.

(c) how it is protected from the OS

  If S3 is enabled, then (b) reserves it from the OS too.

  If S3 is disabled, then the range needs no protection.

(d) how it is accessed on the S3 resume path

  It is rewritten same as in (a), which is fine because (b) reserved it.

(e) how it is accessed on the warm reset path

  It is rewritten same as in (a).

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien@xen.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>