Define DMA address and direction types

Define DMA address and direction types

As a preliminary to adding more extensive DMA and IOMMU infrastructure
support into qemu, this patch defines a dma_addr_t for storing DMA bus
addresses and a DMADirection enum which describes whether a DMA is
from an external device to main memory or from main memory to an
external device.

For now dma_addr_t is just defined to be target_phys_addr_t, but in
future, we can change this to support machines where we have bus
addresses which don't necessarily have the same format as CPU physical
addresses.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>