[libvirt] [RFC] extend USB controller 'model' to support xen pvusb

Hi, List, Currently libxl supports pvusb type USB controller hotplug and in creation time, I'd like to add the support in libvirt libxl driver too. According to current USB controller schema, we need to extend 'model' to support xen PVUSB type. Since libxl support USB controller of version 1.1 and version 2.0, we may add two models: "pvusb1" (pvusb type, version 1.1) and "pvusb2" (pvusb type, version 2.0). Do you have any opinions on that? If it is agreed, I'll try to implement. Thanks, Chunyan