No picture from USB webcam (kvm 0.13-rc1)

I export my Linux webcam to KVM using the switches "-usb -usbdevice host:046d:0929". The XP guest sees the webcam and the drivers install, but the camera only shows a black image. When I open the camera in Windows Explorer, it says "0 images" and a black image, while on a real XP, it says "1 image" and shows the video from the camera. Same problem with different webcams. Same cameras work fine on a native XP install.