OvmfPkg/VmgExitLib: Set the SwScratch valid bit for MMIO events

OvmfPkg/VmgExitLib: Set the SwScratch valid bit for MMIO events

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: c45f678a1ea2080344e125dc55b14e4b9f98483d
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <45ccb63c2dadd834e2c47bf10c9e59c6766d7eb6.1604685192.git.thomas.lendacky@amd.com>