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>