Qemu: Introduce usb hub related test case into virt-test

This pullreq adds or updates some API in qemu_vm/virt_vm modules, in order to support testing with usb-hub device.

I had a simple test, found one failed, but looks it because of some guest hang problem ( bug found? lol)
```
TESTS: 36
(1/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_kbd.usb_default.uhci: PASS (81.68 s)
(2/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_kbd.usb_default.ehci: PASS (98.45 s)
(3/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_kbd.usb_default.xhci: PASS (98.81 s)
(4/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_kbd.with_usb_hub.uhci: PASS (99.32 s)
(5/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_kbd.with_usb_hub.ehci: PASS (95.81 s)
(6/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_kbd.with_usb_hub.xhci: PASS (94.88 s)
(7/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_mouse.usb_default.uhci: PASS (98.42 s)
(8/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_mouse.usb_default.ehci: PASS (97.22 s)
(9/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_mouse.usb_default.xhci: PASS (98.87 s)
(10/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_mouse.with_usb_hub.uhci: PASS (100.56 s)
(11/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_mouse.with_usb_hub.ehci: PASS (99.36 s)
(12/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_mouse.with_usb_hub.xhci: PASS (99.20 s)
(13/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_tablet.usb_default.uhci: PASS (98.96 s)
(14/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_tablet.usb_default.ehci: PASS (93.86 s)
(15/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_tablet.usb_default.xhci: PASS (99.18 s)
(16/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_tablet.with_usb_hub.uhci: PASS (101.39 s)
(17/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_tablet.with_usb_hub.ehci: PASS (97.99 s)
(18/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_tablet.with_usb_hub.xhci: PASS (111.14 s)
(19/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_ccid.usb_default.uhci: PASS (99.22 s)
(20/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_ccid.usb_default.ehci: PASS (99.32 s)
(21/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_ccid.usb_default.xhci: PASS (98.27 s)
(22/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_ccid.with_usb_hub.uhci: PASS (95.07 s)
(23/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_ccid.with_usb_hub.ehci: PASS (99.45 s)
(24/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_ccid.with_usb_hub.xhci: FAIL (414.12 s)
(25/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_audio.usb_default.uhci: PASS (100.02 s)
(26/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_audio.usb_default.ehci: PASS (99.18 s)
(27/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_audio.usb_default.xhci: PASS (97.05 s)
(28/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_audio.with_usb_hub.uhci: PASS (99.34 s)
(29/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_audio.with_usb_hub.ehci: PASS (100.64 s)
(30/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_audio.with_usb_hub.xhci: PASS (142.53 s)
(31/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_hub.usb_default.uhci: PASS (99.10 s)
(32/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_hub.usb_default.ehci: PASS (99.58 s)
(33/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_hub.usb_default.xhci: PASS (97.21 s)
(34/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_hub.with_usb_hub.uhci: PASS (99.45 s)
(35/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_hub.with_usb_hub.ehci: PASS (99.45 s)
(36/36) qcow2.virtio_blk.smp2.virtio_net.Fedora.18.x86_64.usb.usb_boot.usb_hub.with_usb_hub.xhci: PASS (185.41 s)
TOTAL TIME: 3989.65 s (01:06:29)
TESTS PASSED: 35
TESTS FAILED: 1
SUCCESS RATE: 97.22 %
```