OvmfPkg: PciHostBridgeLib: set RootBus->AllocationAttributes

OvmfPkg: PciHostBridgeLib: set RootBus->AllocationAttributes

InitRootBridge() in "OvmfPkg/PciHostBridgeDxe/PciHostBridge.c" passes the
EFI_PCI_HOST_BRIDGE_COMBINE_MEM_PMEM allocation attribute to
RootBridgeConstructor(); we should do the same here.

From "MdePkg/Include/Protocol/PciHostBridgeResourceAllocation.h":

/// If this bit is set, then the PCI Root Bridge does not support separate
/// windows for Non-prefetchable and Prefetchable memory. A PCI bus driver
/// needs to include requests for Prefetchable memory in the
/// Non-prefetchable memory pool.

Which implies that both the 32-bit and 64-bit prefetchable MMIO apertures
should be marked empty. (The CreateRootBridge() function actually enforces
this in "MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c".)

Furthermore, since OvmfPkg/PciHostBridgeDxe does *not* set the
EFI_PCI_HOST_BRIDGE_MEM64_DECODE allocation attribute:

/// If this bit is set, then the PCI Root Bridge supports 64 bit memory
/// windows.  If this bit is not set, the PCI bus driver needs to include
/// requests for 64 bit memory address in the corresponding 32 bit memory
/// pool.

we follow suit in the PciHostBridgeLib instance.

In turn, the 64-bit MMIO apertures (both prefetchable and
non-prefetchable) should be marked empty.
MdeModulePkg/Bus/Pci/PciHostBridgeDxe enforces this too.

(64-bit MMIO aperture support, based on yet more fw_cfg files, is a
planned future improvement.)

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>