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 
in fact, The function utils_test.run_autotest()  will run an autotest control file inside a guest (linux only utility).
and will start iozone or stress process.
but the time-consuming of it depends on the performance of guest.
sometimes, due to the bad capability of guest， the process of process couldn't be started right now，
which will lead to utils_misc.wait_for() return False.
at last , the case for iozone or stress test will fail.

so i add a delay to this avoid situation.
of course, it's not mandatory for user, who can configure it by themselves.