OvmfPkg/MemEncryptSevLib: Add an SEV-ES guest indicator function

OvmfPkg/MemEncryptSevLib: Add an SEV-ES guest indicator function

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198

Create a function that can be used to determine if the VM is running
as an SEV-ES guest.

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>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>