xl: add pvusb commands

xl: add pvusb commands

Add pvusb commands: usbctrl-attach, usbctrl-detach, usb-list,
usbdev-attach and usbdev-detach.

To attach a usb device to guest through pvusb, one could follow
following example:

 #xl usbctrl-attach test_vm version=1 ports=8

 #xl usb-list test_vm
 will show the usb controllers and port usage under the domain.

 #xl usbdev-attach test_vm hostbus=1 hostaddr=2
 will find the first usable controller:port, and attach usb
 device whose busnum is 1 and devnum is 6.
 One could also specify which <controller> and which <port>.

 #xl usbdev-detach test_vm 0 1
 will detach USB device under controller 0 port 1.

 #xl usbctrl-detach test_vm dev_id
 will destroy the controller with specified dev_id. Dev_id
 can be traced in usb-list info.

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>