OvmfPkg: PlatformPei: lifecycle fixes for the LockBox area

OvmfPkg: PlatformPei: lifecycle fixes for the LockBox area

If (mBootMode == BOOT_ON_S3_RESUME) -- that is, we are resuming --, then
the patch has no observable effect.

If (mBootMode != BOOT_ON_S3_RESUME && mS3Supported) -- that is, we are
booting or rebooting, and S3 is supported), then the patch has no
observable effect either.

If (mBootMode != BOOT_ON_S3_RESUME && !mS3Supported) -- that is, we are
booting or rebooting, and S3 is unsupported), then the patch effects the
following two fixes:

- The LockBox storage is reserved from DXE (but not the OS). Drivers in
  DXE may save data in the LockBox regardless of S3 support, potentially
  corrupting any overlapping allocations. Make sure there's no overlap.

- The LockBox storage is cleared. A LockBox inherited across a non-resume
  reboot, populated with well-known GUIDs, breaks drivers that want to
  save entries with those GUIDs.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Matt Fleming <matt.fleming@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15418 6f19259b-4bc3-4df7-8a09-765794883524