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>

OvmfPkg/Include/Library/MemEncryptSevLib.h

@@ -13,6 +13,18 @@
 
 #include <Base.h>
 
+/**
+  Returns a boolean to indicate whether SEV-ES is enabled.
+
+  @retval TRUE           SEV-ES is enabled
+  @retval FALSE          SEV-ES is not enabled
+**/
+BOOLEAN
+EFIAPI
+MemEncryptSevEsIsEnabled (
+  VOID
+  );
+
 /**
   Returns a boolean to indicate whether SEV is enabled