Using USB devices on KVM guests

Hi,
I found in Internet that KVM supports USB host devices sharing and i've launched the command to do this in the QEMU console. It works but i don't know how to make it at the start of the Vm.
I also would to use the KVM's "auto-detect USB" feature ( http://www.linux-kvm.com/content/kvm-74-will-support-usb-device-automatic-connect-and-disconnect for infos), but when i try to unplug and re-plug a device, guest OS doesn't load it.

Thanks

see 'man qm' and use the search function in this forum, there are several threads about usb.