Patch sets for usb hotplug function

1) libvirt_bench: Add stress tests for usb-device hotplug function.
2) libvirt: add tests for controller involving usb-device hotplug.
3) libvirt: add tests for usb-device hotplug function
    The test target device includes keyboard, mouse, tablet, and disk.
@yangdongsheng 
I have updated it as your comment.
Thanks.