add pxb support for libvirt

The NUMA information for a PCIe device is determined by the bus to which
the device is connected, not by the device itself. In the current nova
implementation, all devices are connected to the Root Bus, which has
no NUMA information, so NUMA information is omitted.

This blueprint solves this by adding a PXB (PCIe Expander Bus) for each
NUMA node as a bus with NUMA information.