OvmfPkg/AmdSevDxe: decrypt the pages of the initial SMRAM save state map

OvmfPkg/AmdSevDxe: decrypt the pages of the initial SMRAM save state map

Based on the following patch from Brijesh Singh <brijesh.singh@amd.com>:

  [PATCH v2 1/2] OvmfPkg/AmdSevDxe: Clear the C-bit from SMM Saved State
  http://mid.mail-archive.com/20180228161415.28723-2-brijesh.singh@amd.com
  https://lists.01.org/pipermail/edk2-devel/2018-February/022016.html

Original commit message from Brijesh:

> When OVMF is built with SMM, SMMSaved State area (SMM_DEFAULT_SMBASE +
> SMRAM_SAVE_STATE_MAP_OFFSET) contains data which need to be accessed by
> both guest and hypervisor. Since the data need to be accessed by both
> hence we must map the SMMSaved State area as unencrypted (i.e C-bit
> cleared).
>
> This patch clears the SavedStateArea address before SMBASE relocation.
> Currently, we do not clear the SavedStateArea address after SMBASE is
> relocated due to the following reasons:
>
> 1) Guest BIOS never access the relocated SavedStateArea.
>
> 2) The C-bit works on page-aligned address, but the SavedStateArea
> address is not a page-aligned. Theoretically, we could roundup the
> address and clear the C-bit of aligned address but looking carefully we
> found that some portion of the page contains code -- which will causes a
> bigger issue for the SEV guest. When SEV is enabled, all the code must
> be encrypted otherwise hardware will cause trap.

Changes by Laszlo:

- separate AmdSevDxe bits from SmmCpuFeaturesLib bits;

- spell out PcdLib dependency with #include and in LibraryClasses;

- replace (SMM_DEFAULT_SMBASE + SMRAM_SAVE_STATE_MAP_OFFSET) calculation
  with call to new MemEncryptSevLocateInitialSmramSaveStateMapPages()
  function;

- consequently, pass page-aligned BaseAddress to
  MemEncryptSevClearPageEncMask();

- zero the pages before clearing the C-bit;

- pass Flush=TRUE to MemEncryptSevClearPageEncMask();

- harden the treatment of MemEncryptSevClearPageEncMask() failure.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>