usb: Fix usb_packet_map() in the presence of IOMMUs

With the IOMMU infrastructure introduced before 1.2, we need to use
dma_memory_map() to obtain a qemu pointer to memory from an IO bus address.
However, dma_memory_map() alters the given length to reflect the length
over which the used DMA translation is valid - which could be either more
or less than the requested length.

usb_packet_map() does not correctly handle these cases, simply failing if
dma_memory_map() alters the requested length.  If dma_memory_map()
increased the length, we just need to use the requested length for the
qemu_iovec_add().  However, if it decreased the length, it means that a
single DMA translation is not valid for the whole sglist element, and so
we need to loop, splitting it up into multiple iovec entries for each
piece with a DMA translation (in practice >2 pieces is unlikely).

This patch implements the correct behaviour

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
