OvmfPkg: PciHostBridgeLib: clear RootBus->DmaAbove4G

OvmfPkg: PciHostBridgeLib: clear RootBus->DmaAbove4G

When this BOOLEAN member is FALSE, and the caller tries to set up a DMA
transfer between a PCI device and a host buffer not entirely under 4GB,
then "MdeModulePkg/Bus/Pci/PciHostBridgeDxe" sets up a bounce buffer under
4GB, in the implementation of EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL.Map().

Since that's exactly what RootBridgeIoMap() does in
"OvmfPkg/PciHostBridgeDxe/PciRootBridgeIo.c", stick with it in this
conversion.

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: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>