OvmfPkg: switch to MdeModulePkg/Bus/Pci/PciHostBridgeDxe

OvmfPkg: switch to MdeModulePkg/Bus/Pci/PciHostBridgeDxe

The old driver is retained for now; it remains available with "-D
USE_OLD_PCI_HOST". This is because I'd like to involve end users and
downstreams in testing the new drier, but also allow them to switch back
to the old driver at the first sight of trouble, while we debug the new
driver in parallel.

In a few weeks the ifdeffery and the "OvmfPkg/PciHostBridgeDxe/" driver
should be removed.

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