From KVM

After adding usb device,when I use lsusb, I can see my webcam listed but nothing under /proc/bus/usb/ there is nothing?[edit]

mount usbfs, mount -t usbfs none /proc/bus/usb. You can chown them to your user if you're not running as root.

can one connect/disconnect usb devices, and XP manages this without manual "mount" intervention from host os?[edit]

usb is possible. for pci, it needs extra work (Januar 2008), check qemu's usage documentation on usb.