iommu: Allow PCI to use IOMMU infrastructure

This patch adds some hooks to let PCI devices and busses use the new IOMMU
infrastructure.  When IOMMU support is enabled, each PCI device now
contains a DMAContext * which is used by the pci_dma_*() wrapper functions.

By default, the contexts are initialized to NULL, assuming no IOMMU.
However the platform or host bridge code which sets up the PCI bus can use
pci_setup_iommu() to set a function which will determine the correct
DMAContext for a given PCI device.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
