Add support for USB controller for OpenStack Compute (nova)

Currently nova doesn't support to create USB controller for VM, so libvirt/qemu create a default USB 1.1 controller. If users want to use a USB 2.0 device, it may speed mismatched with the default controller. So I think its necessary for nova to support to create USB controller for VM.

Support passthrough of USB device for OpenStack Compute (nova)

Currently, nova has supported feature of pci-passthrough, but hasn't supported usb-passthrough. There were a bp about it in https://blueprints.launchpad.net/nova/+spec/host-usb-passthrough but don't have progress by now. As I know, some telecom and enterprise customers have requirement of usb-passthrough especiall...