This patch adds functions to pci.[ch] to perform PCI DMA operations.
At present, these are just stubs which perform directly cpu physical
memory accesses.  Stubs are included which are analogous to
cpu_physical_memory_{read,write}(), the stX_phys() and ldX_phys()
functions and cpu_physical_memory_{map,unmap}().

In addition, a wrapper around qemu_sglist_init() is provided, which
also takes a PCIDevice *.  It's assumed that _init() is the only
sglist function which will need wrapping, the idea being that once we
have IOMMU support whatever IOMMU context handle the wrapper derives
from the PCI device will be stored within the sglist structure for
later use.

Using these stubs, however, distinguishes PCI device DMA transactions from
other accesses to physical memory, which will allow PCI IOMMU support to
be added in one place, rather than updating every PCI driver at that time.

That is, it allows us to update individual PCI drivers to support an IOMMU
without having yet determined the details of how the IOMMU emulation will
operate.  This will let us remove the most bitrot-sensitive part of an
IOMMU patch in advance.
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>