OvmfPkg: Add support to perform SEV-ES initialization

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



When SEV-ES is enabled, then SEV is also enabled. Add support to the SEV
initialization function to also check for SEV-ES being enabled, and if
enabled, set the SEV-ES enabled PCD (PcdSevEsIsEnabled).

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>