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.
This is what we want to solve by ```utils_misc.wait_for(_is_stress_running, timeout=120):```. It will wait 120s for the iozone process running in guest. why you need another delay for it?