usb-xhci: Use PCI DMA helper functions

Shortly before 1.0, we added helper functions / wrappers for doing PCI DMA
from individual devices.  This makes what's going on clearer and means that
when we add IOMMU support somewhere in the future, only the general PCI
code will have to change, not every device that uses PCI DMA.

However, usb-xhci is not using these wrappers, despite being a PCI only
device.  This patch remedies the situation, using the pci dma functions
instead of direct calls to cpu_physical_memory_{read,write}().  Likewise
address parameters for DMA are changed to dma_addr_t instead of
target_phys_addr_t.

[ kraxel: removed #ifdefs ]
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>