OvmfPkg: PciHostBridgeLib: set supported and initial attributes in RootBus

OvmfPkg: PciHostBridgeLib: set supported and initial attributes in RootBus

These settings are copied from the RootBridgeConstructor() function, file
"OvmfPkg/PciHostBridgeDxe/PciRootBridgeIo.c".

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>