May be you need to have root Permissions to use usb-devices.
For example if you want to use an usb-stick with windows emulated on qemu-kvm
you need first to login as superuser.
Then, start terminal and type 'lsusb' to detect what for a bus and what for a device
you must use to access an usb-stick with an emulated operating system.
If you want to get only informations you can use 'lsusb' without root permissions.
I don't know if you can change the permissions with chmod for the device, but it is
possible that the permissions will change to the former status after a new start of
the system.
