OvmfPkg/FvbServicesSmm: use the VmgExitLibNull

OvmfPkg/FvbServicesSmm: use the VmgExitLibNull

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3835

The commit ade62c18f4742301bbef474ac10518bde5972fba caused a boot failure
when OVMF is build with SECURE_BOOT/SMM enabled.

This happen because the above commit extended the BaseMemEncryptSevLib.inf
to include VmgExitLib. The FvbServicesSmm uses the functions provided
by the MemEncryptSevLib to clear the memory encryption mask from the
page table. It created a dependency, as shown below

OvmfPkg/FvbServicesSmm.inf
 ---> MemEncryptSevLib                   class
 ---> "OvmfPkg/BaseMemEncryptSevLib/DxeMemEncryptSevLib.inf"  instance
 ---> VmgExitLib
 ---> "OvmfPkg/VmgExitLib"    instance
 ---> LocalApicLib                       class
 ---> UefiCpuPkg/BaseXApicX2ApicLib/BaseXApicX2ApicLib.inf    instance
 ---> TimerLib                           class
 ---> "OvmfPkg/AcpiTimerLib/DxeAcpiTimerLib.inf"              instance
 ---> PciLib                             class
 ---> "OvmfPkg/DxePciLibI440FxQ35/DxePciLibI440FxQ35.inf"     instance

The LocalApicLib provides a constructor, execution of the constructor
causes an exception. The SEV-ES and SEV-SNP do not support the SMM, so
skip including the VmgExitLib chain. Use the module override to use the
VmgExitLibNull to avoid the inclusion of unneeded LocalApicLib dependency
chain in FvbServicesSmm. We ran similar issue for AmdSevDxe driver,
see commit 19914edc5a0202cc7830f819ffac7e7b2368166a

After the patch, the dependency look like this:

OvmfPkg/FvbServicesSmm.inf
 ---> MemEncryptSevLib                   class
 ---> "OvmfPkg/BaseMemEncryptSevLib/DxeMemEncryptSevLib.inf"  instance
 ---> VmgExitLib
 ---> "UefiCpuPkg/Library/VmgExitLibNull"    instance

Fixes: ade62c18f4742301bbef474ac10518bde5972fba
Reported-by: Aaron Young <aaron.young@oracle.com>
Cc: Dann Frazier <dann.frazier@canonical.com>
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>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Tested-by: dann frazier <dann.frazier@canonical.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>