libxl: add new pvusb backend "qusb" provided by qemu
Add a new pvusb backend type "qusb" which is provided by qemu. It can
be selected either by specifying the type directly in the configuration
or it is selected automatically by libxl in case there is no "usbback"
driver loaded.
The "qusb" backend is a full replacement for the kernel based backend
"vusb". The interface to the frontend is the very same including all
Xenstore paths, while the backend paths in Xenstore differ ("qusb"
instead ov "vusb").
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>