OvmfPkg/PlatformPei: set PCI IO port aperture dynamically

OvmfPkg/PlatformPei: set PCI IO port aperture dynamically

Make PcdPciIoBase and PcdPciIoSize dynamic PCDs, and set them in
MemMapInitialization(), where we produce our EFI_RESOURCE_IO descriptor
HOB. (The PCD is consumed by the core PciHostBridgeDxe driver, through our
PciHostBridgeLib instance.)

Take special care to keep the GCD IO space map unchanged on all platforms
OVMF runs on.

Cc: Gabriel Somlo <somlo@cmu.edu>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1333238
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>