OvmfPkg: PciHostBridgeLib: initialize RootBus->DevicePath

OvmfPkg: PciHostBridgeLib: initialize RootBus->DevicePath

We copy the code from InitRootBridge()
[OvmfPkg/PciHostBridgeDxe/PciHostBridge.c], with a slight change: the
device path is allocated separately now.

This is the final field to initialize in PCI_ROOT_BRIDGE.

The type EFI_PCI_ROOT_BRIDGE_DEVICE_PATH is renamed to
OVMF_PCI_ROOT_BRIDGE_DEVICE_PATH. The original is a misnomer (it is not a
standard UEFI type) that dates back to PcAtChipsetPkg/PciHostBridgeDxe.
Simply removing the EFI_ suffix would result in
PCI_ROOT_BRIDGE_DEVICE_PATH, where PCI_ could incorrectly suggest a
relation with the PCI standards or the PCI-related generic edk2 code.

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>