OvmfPkg: PlatformPei: set SMBIOS entry point version dynamically

Git commit 54753b60 (SVN r16870), "MdeModulePkg: Update SMBIOS revision to
3.0." changed PcdSmbiosVersion from 0x0208 to 0x0300. This controls the
version number of the SMBIOS entry point table (and other things) that
"MdeModulePkg/Universal/SmbiosDxe" installs.

Alas, this change breaks older Linux guests, like RHEL-6 (up to RHEL-6.7);
those are limited to 2.x (both in the guest kernel firmware driver, and in
the dmidecode utility). The SMBIOS 3.0 entry point has a different GUID --
defined in UEFI 2.5 -- pointing to it in the UEFI Configuration Table, and
guest kernels that lack upstream kernel commit e1ccbbc9d5 don't recognize
it.

The v2.1.0+ machine types of QEMU generate SMBIOS payload for the firmware
to install. The payload includes the entry point table ("anchor" table).
OvmfPkg/SmbiosPlatformDxe cannot install the anchor table (because that is
the jurisdiction of the generic "MdeModulePkg/Universal/SmbiosDxe"
driver); however, we can parse the entry point version from QEMU's anchor
table, and instruct "MdeModulePkg/Universal/SmbiosDxe" to adhere to that
version.

On machine types older than v2.1.0, the feature is not available, but
then, should anything in OVMF install SMBIOS tables, version 2.8 is simply
safer / more widely supported than 3.0 -- hence the default 2.8 value for
the dynamic PCD.

We set the PCD in PlatformPei (when not on the S3 resume path), because
that's an easy and certain way to set the PCD before a DXE driver reads
it. This follows the example of PcdEmuVariableNvStoreReserved (which is
read by EmuVariableFvbRuntimeDxe).

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1232876
Cc: Gabriel Somlo <somlo@cmu.edu>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Gabriel Somlo <somlo@cmu.edu>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17676 6f19259b-4bc3-4df7-8a09-765794883524

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