OvmfPkg/MemEncryptSevLib: add MemEncryptSevSnpEnabled()

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3275

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

Cc: Michael Roth <michael.roth@amd.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>

OvmfPkg/Include/Library/MemEncryptSevLib.h

@@ -47,6 +47,18 @@ typedef enum {
   MemEncryptSevAddressRangeError,
 } MEM_ENCRYPT_SEV_ADDRESS_RANGE_STATE;
 
+/**
+  Returns a boolean to indicate whether SEV-SNP is enabled
+
+  @retval TRUE           SEV-SNP is enabled
+  @retval FALSE          SEV-SNP is not enabled
+**/
+BOOLEAN
+EFIAPI
+MemEncryptSevSnpIsEnabled (
+  VOID
+  );
+
 /**
   Returns a boolean to indicate whether SEV-ES is enabled.