OvmfPkg: match PCI config access to machine type (if not USE_OLD_PCI_HOST)

OvmfPkg: match PCI config access to machine type (if not USE_OLD_PCI_HOST)

If USE_OLD_PCI_HOST is FALSE, then we switch all executable module types
supported by DxePciLibI440FxQ35 to the following library instance stack:

  BasePciSegmentLibPci  [class: PciSegmentLib]
    DxePciLibI440FxQ35  [class: PciLib]
      BasePciCf8Lib     [class: PciCf8Lib]
      BasePciExpressLib [class: PciExpressLib]

Every module will select 0xCF8 vs. ECAM based on the OVMF platform type
(i440fx or Q35). Notably, MdeModulePkg/Bus/Pci/PciHostBridgeDxe is among
the affected drivers.

The BasePciExpressLib instance is where the PcdPciExpressBaseAddress PCD
fills its original role.

Cc: Gabriel Somlo <somlo@cmu.edu>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Michał Zegan <webczat_200@poczta.onet.pl>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Tested-by: Gabriel Somlo <somlo@cmu.edu>
Tested-by: Michał Zegan <webczat_200@poczta.onet.pl>