Convert into trace event. Otherwise the message
	dma: unregistered DMA channel used nchan=0 dma_pos=0 dma_len=1
gets printed every time and fills up the log-file with 50 MiB / minute.
Signed-off-by: NPhilipp Hahn <hahn@univention.de>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>