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.
In the process of test,
I found that, if i call utils_misc.wait_for() right now when run_autotest() returns.
The function _is_stress_running will  waste away the (timeout=120) and  return false. 
I just suspect that the run_autotest() just return, not finished doing some work.
To be honest，I haven't investigated it carefully.
so i take the simple method， just wait for some seconds.