OvmfPkg: clone PciHostBridgeLib from MdeModulePkg's Null instance

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>