qemu.tests.usb: Make cmds in usb_hotplug test configurable

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests.cfg: Make usb_hotplug work on win guest

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Merge pull request #550 from zhouqt/usb

qemu: Make usb_hotplug test work on Win guests