OvmfPkg: clone PciHostBridgeLib from MdeModulePkg's Null instance

In this patch we clone "MdeModulePkg/Library/PciHostBridgeLibNull" for
customization under OvmfPkg. Differences relative to a verbatim copy:

- the Null suffix is dropped from file names,
- the UNI file is dropped, together with the corresponding MODULE_UNI_FILE
  reference in the INF file,
- the INF file receives a new FILE_GUID,
- the top comments in the files mention OVMF, not a null instance.

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>

OvmfPkg/Library/PciHostBridgeLib/PciHostBridgeLib.inf

@@ -0,0 +1,38 @@
+## @file
+#  OVMF's instance of the PCI Host Bridge Library.
+#
+#  Copyright (C) 2016, Red Hat, Inc.
+#  Copyright (c) 2016, Intel Corporation. All rights reserved.<BR>
+#
+#  This program and the accompanying materials are licensed and made available
+#  under the terms and conditions of the BSD License which accompanies this
+#  distribution. The full text of the license may be found at
+#  http://opensource.org/licenses/bsd-license.php
+#  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR
+#  IMPLIED.
+#
+#
+##
+
+[Defines]
+  INF_VERSION                    = 0x00010005
+  BASE_NAME                      = PciHostBridgeLib
+  FILE_GUID                      = 9F2BC05E-51EA-4AED-9A3E-7699641734E8
+  MODULE_TYPE                    = DXE_DRIVER
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = PciHostBridgeLib
+
+#
+# The following information is for reference only and not required by the build
+# tools.
+#
+#  VALID_ARCHITECTURES           = IA32 X64 IPF EBC
+#
+
+[Sources]
+  PciHostBridgeLib.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+  MdeModulePkg/MdeModulePkg.dec