qemu: Make usb_hotplug test work on Win guests

I made some simple tests, both linux and windows guests looks work well.

```
(1/6) Win8.usb.usb_hotplug.usb_normal_test.usb_kbd.usb_default.uhci: PASS (146.83 s)
(2/6) Win8.usb.usb_hotplug.usb_normal_test.usb_mouse.usb_default.uhci: PASS (63.02 s)
(3/6) Win8.usb.usb_hotplug.usb_normal_test.usb_tablet.usb_default.uhci: PASS (61.87 s)
(4/6) Win8.usb.usb_hotplug.usb_normal_test.usb_ccid.usb_default.uhci: FAIL (52.72 s)
(5/6) Win8.usb.usb_hotplug.usb_normal_test.usb_audio.usb_default.uhci: FAIL (50.60 s)
(6/6) Win8.usb.usb_hotplug.usb_normal_test.usb_hub.usb_default.uhci: PASS (60.92 s)

(1/6) Win7.sp0.usb.usb_hotplug.usb_normal_test.usb_kbd.usb_default.uhci: PASS (67.59 s)
(2/6) Win7.sp0.usb.usb_hotplug.usb_normal_test.usb_mouse.usb_default.uhci: PASS (63.17 s)
(3/6) Win7.sp0.usb.usb_hotplug.usb_normal_test.usb_tablet.usb_default.uhci: PASS (62.08 s)
(4/6) Win7.sp0.usb.usb_hotplug.usb_normal_test.usb_ccid.usb_default.uhci: PASS (58.12 s)
(5/6) Win7.sp0.usb.usb_hotplug.usb_normal_test.usb_audio.usb_default.uhci: PASS (62.35 s)
(6/6) Win7.sp0.usb.usb_hotplug.usb_normal_test.usb_hub.usb_default.uhci: PASS (63.05 s)

(1/6) Fedora.18.usb.usb_hotplug.usb_normal_test.usb_kbd.usb_default.uhci: PASS (40.25 s)
(2/6) Fedora.18.usb.usb_hotplug.usb_normal_test.usb_mouse.usb_default.uhci: PASS (40.39 s)
(3/6) Fedora.18.usb.usb_hotplug.usb_normal_test.usb_tablet.usb_default.uhci: PASS (40.30 s)
(4/6) Fedora.18.usb.usb_hotplug.usb_normal_test.usb_ccid.usb_default.uhci: PASS (38.13 s)
(5/6) Fedora.18.usb.usb_hotplug.usb_normal_test.usb_audio.usb_default.uhci: PASS (40.06 s)
(6/6) Fedora.18.usb.usb_hotplug.usb_normal_test.usb_hub.usb_default.uhci: PASS (41.00 s)
```
There are 2 failures (usb_ccid, usb_audio) when testing Win8, the guest system can't recognize these 2 devices... I'll contact the qemu developer to check this problem.