OvmfPkg: introduce PcdQemuSmbiosValidated

OvmfPkg: introduce PcdQemuSmbiosValidated

This dynamic PCD will enable a small code de-duplication between
OvmfPkg/SmbiosPlatformDxe and OvmfPkg/Library/SmbiosVersionLib. Since both
of those are also used in ArmVirtQemu.dsc, and we should avoid
cross-package commits when possible, this patch declares
PcdQemuSmbiosValidated first, and sets defaults for it in the OvmfPkg DSC
files.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Wei Huang <wei@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Gabriel L. Somlo <somlo@cmu.edu>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18178 6f19259b-4bc3-4df7-8a09-765794883524