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.
sleep is not a good idea. We prefer wair_for(). If adding a delay before wait_for() will make it working well, then increase the timeout in wait_for() can get the same goal I think.