OvmfPkg/PlatformBootManagerLib: Handle the absence of PCI bus on Xen PVH

OvmfPkg/PlatformBootManagerLib: Handle the absence of PCI bus on Xen PVH

When running in a Xen PVH guest, there's nothing to do in
PciAcpiInitialization() because there isn't any PCI bus. When the Host
Bridge DID isn't recognised, simply continue. (The value of
PcdOvmfHostBridgePciDevId would be 0 because it isn't set.)

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1689
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190813113119.14804-29-anthony.perard@citrix.com>