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

OvmfPkg/VmgExitLib: Set the SwScratch valid bit for IOIO 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: 0020157a9825e5f5784ff014044f11c0558c92fe
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: <f817d034cea37fa78e00e86f61c3445f1208226d.1604685192.git.thomas.lendacky@amd.com>