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 fact, i haven't found any official documnet mentioned the maxi value about hotplug。
i get it by the practice on test machine.

Maybe the value depends on different OS , machine , or other unknown factor.
but it doesn't matter.
The reason is that the value is set configurable in the config file.
User can adjust it by themselves。