dbdma: warn when using unassigned channel

With this, it's easier to know if a guest uses an invalid and/or unimplemented
DMA channel.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
