OvmfPkg/VmgExitLib: Set the SW exit fields when performing VMGEXIT

OvmfPkg/VmgExitLib: Set the SW exit fields when performing VMGEXIT

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 bits for the
software exit information fields when performing a VMGEXIT (SwExitCode,
SwExitInfo1, SwExitInfo2).

Fixes: 61bacc0fa16fd6f595a2c4222425cb6286e19977
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: <986e157c13bf33e529b1d16ab1b52e99a74a734f.1604685192.git.thomas.lendacky@amd.com>