Comment # 10 on bug 1179744 from Takashi Iwai

Thanks, this explains why no usb device appears.  The kernel failed to get the
USB descriptor.

I'm not sure now whether it's a bug in IOMMU or rather in USB controller. 
Judging from the dmseg output, it's handled by ohci-pci.

Adding both USB and IOMMU guys to Cc for better clues.