pci: use memory core for iommu support

Use the new iommu support in the memory core for iommu support.  The only
user, spapr, is also converted, but it still provides a DMAContext
interface until the non-PCI bits switch to AddressSpace.

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Avi Kivity <avi.kivity@gmail.com>
[ Do not calls memory_region_del_subregion() on the device's
  bus_master_enable_region, it is an alias; return an AddressSpace
  from the IOMMU hook and remove the destructor hook. - David Gibson ]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>