Ovmfpkg: Prepare OvmfPkg to use the AmdSvsmLib library

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4654

The MpInitLib library will be updated to use the new AmdSvsmLib library.
To prevent any build breakage, update the OvmfPkg DSCs file to include
the AmdSvsmLib NULL library.

Cc: Anatol Belski <anbelski@linux.microsoft.com>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Corvin Köhne <corvink@freebsd.org>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Jianyong Wu <jianyong.wu@arm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Michael Roth <michael.roth@amd.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Rebecca Cran <rebecca@bsdio.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>

OvmfPkg/OvmfXen.dsc

@@ -233,6 +233,7 @@
 !endif
 
 [LibraryClasses.common]
+  AmdSvsmLib|UefiCpuPkg/Library/AmdSvsmLibNull/AmdSvsmLibNull.inf
   BaseCryptLib|CryptoPkg/Library/BaseCryptLib/BaseCryptLib.inf
   CcExitLib|UefiCpuPkg/Library/CcExitLibNull/CcExitLibNull.inf
   TdxLib|MdePkg/Library/TdxLib/TdxLib.inf