ide/ahci: Use universal DMA helper functions

ide/ahci: Use universal DMA helper functions

The AHCI device can provide both PCI and SysBus AHCI device
emulations.  For this reason, it wasn't previously converted to use
the pci_dma_*() helper functions.  Now that we have universal DMA
helper functions, this converts AHCI to use them.

The DMAContext is obtained from pci_dma_context() in the PCI case and
set to NULL in the SysBus case (i.e. we assume for now that a SysBus
AHCI has no IOMMU translation).

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>