OvmfPkg: PciHostBridgeLib: set RootBus->Segment

OvmfPkg: PciHostBridgeLib: set RootBus->Segment

This is the first of the patches that set the fields of PCI_ROOT_BRIDGE.
The structure is zero-filled as a precaution for later field additions.

Here we set the Segment member explicitly to zero (so that any later
customization can be easier).

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>