OvmfPkg: factor the IO aperture shared by all PCI root bridges into PCDs

OvmfPkg: factor the IO aperture shared by all PCI root bridges into PCDs

At the moment we don't intend to customize this aperture at runtime, but
going forward, two modules will need to know about it: PlatformPei (as
before), and OVMF's upcoming PciHostBridgeLib instance (because the core
PciHostBridgeDxe driver requires the library to state the exact apertures
for all root bridges).

On QEMU, all root bridges share the same IO port aperture, hence one pair
of PCDs suffices.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>