OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Set the SwScratch valid bit

OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Set the SwScratch valid bit

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3008

All fields that are set in the GHCB should have their associated bit in
the GHCB ValidBitmap field set. Add support to set the bit for the scratch
area field (SwScratch).

Fixes: 437eb3f7a8db7681afe0e6064d3a8edb12abb766
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <cc8c8449740d2be0b287e6c69d48bf6cb067c7d8.1604685192.git.thomas.lendacky@amd.com>