OvmfPkg/AmdSevDxe: do not use extended PCI config space

OvmfPkg/AmdSevDxe: do not use extended PCI config space

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

Commit 85b8eac59b8c5bd9c7eb9afdb64357ce1aa2e803 added support to ensure
that MMIO is only performed against the un-encrypted memory. If MMIO
is performed against encrypted memory, a #GP is raised.

The AmdSevDxe uses the functions provided by the MemEncryptSevLib to
clear the memory encryption mask from the page table. If the
MemEncryptSevLib is extended to include VmgExitLib then depedency
chain will look like this:

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

The LocalApicLib provides a constructor that gets called before the
AmdSevDxe can clear the memory encryption mask from the MMIO regions.

When running under the Q35 machine type, the call chain looks like this:

AcpiTimerLibConstructor ()  [AcpiTimerLib]
  PciRead32 ()              [DxePciLibI440FxQ35]
   PciExpressRead32 ()      [PciExpressLib]

The PciExpressRead32 () reads the MMIO region. The MMIO regions are not
yet mapped un-encrypted, so the check introduced in the commit
85b8eac59b8c5bd9c7eb9afdb64357ce1aa2e803 raises a #GP.

The AmdSevDxe driver does not require the access to the extended PCI
config space. Accessing a normal PCI config space, via IO port should be
sufficent. Use the module-scope override to make the AmdSevDxe use the
BasePciLib instead of BasePciExpressLib so that PciRead32 () uses the
IO ports instead of the extended config space.

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>
Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>