libxl: domcreate: support pvusb in configuration file

libxl: domcreate: support pvusb in configuration file

Add code to support pvusb in domain config file. One could specify
usbctrl and usb in domain's configuration file and create domain,
then usb controllers will be created and usb device would be attached
to guest automatically.

One could specify usb controllers and usb devices in config file
like this:
usbctrl=['version=2,ports=4', 'version=1, ports=4', ]
usbdev=['hostbus=2, hostaddr=1, controller=0,port=1', ]

Signed-off-by: Chunyan Liu <cyliu@suse.com>
Signed-off-by: Simon Cao <caobosimon@gmail.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>