Does not add usb-host devices as they are hotplugged

A commandline such as

qemu-kvm -device usb-ehci,id=USBCtrl -device host-usb,bus=USBCtrl.0,hostbus=3

should automatically add all devices on the given bus (here: 3) not only initially, but also when new devices appear on that bus while Qemu runs. Currently, all devices on the bus are added initially, but new devices which are added to the (host) usb while Qemu runs have to be added manually.