qemu.tests.usb_hotplug: Do not need to verify usb device on guest for negative test

Do not need to verify usb device is pluged/unpluged on guest for negative test.

Signed-off-by: Shuping Cui <scui@redhat.com>
Merge pull request #48 from spcui/usb_hotplug

qemu.tests.usb_hotplug: Do not need to verify usb device on guest for negative test