OvmfPkg: PlatformPei: enable PCIEXBAR (aka MMCONFIG / ECAM) on Q35

OvmfPkg: PlatformPei: enable PCIEXBAR (aka MMCONFIG / ECAM) on Q35

The comments in the code should speak for themselves; here we note only
two facts:

- The PCI config space writes (to the PCIEXBAR register) are performed
  using the 0xCF8 / 0xCFC IO ports, by virtue of PciLib being resolved to
  BasePciLibCf8. (This library resolution will permanently remain in place
  for the PEI phase.)

- Since PCIEXBAR counts as a chipset register, it is the responsibility of
  the firmware to reprogram it at S3 resume. Therefore
  PciExBarInitialization() is called regardless of the boot path. (Marcel
  recently posted patches for SeaBIOS that implement this.)

This patch suffices to enable PCIEXBAR (and the dependent ACPI table
generation in QEMU), for the sake of "PCIeHotplug" in the Linux guest:

  ACPI: MCFG 0x000000007E17F000 00003C
        (v01 BOCHS  BXPCMCFG 00000001 BXPC 00000001)
  PCI: MMCONFIG for domain 0000 [bus 00-ff] at [mem 0x80000000-0x8fffffff]
       (base 0x80000000)
  PCI: MMCONFIG at [mem 0x80000000-0x8fffffff] reserved in E820
  acpi PNP0A08:00: _OSC: OS supports
                   [ExtendedConfig ASPM ClockPM Segments MSI]
  acpi PNP0A08:00: _OSC: OS now controls
                   [PCIeHotplug PME AER PCIeCapability]

In the following patches, we'll equip the core PCI host bridge / root
bridge driver and the rest of DXE as well to utilize ECAM on Q35.

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>
Ref: https://github.com/tianocore/edk2/issues/32
Ref: http://thread.gmane.org/gmane.comp.bios.coreboot.seabios/10548
Suggested-by: Marcel Apfelbaum <marcel@redhat.com>
Reported-by: Michał Zegan <webczat_200@poczta.onet.pl>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Tested-by: Gabriel Somlo <somlo@cmu.edu>
Tested-by: Michał Zegan <webczat_200@poczta.onet.pl>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>