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

[Chun Yan Liu]

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