spapr_pci: Allow PCI host bridge DMA window to be configured

spapr_pci: Allow PCI host bridge DMA window to be configured

At present the PCI host bridge (PHB) for the pseries machine type has a
fixed DMA window from 0..1GB (in PCI address space) which is mapped to real
memory via the PAPR paravirtualized IOMMU.

For better support of VFIO devices, we're going to want to allow for
different configurations of the DMA window.

Eventually we'll want to allow the guest itself to reconfigure the window
via the PAPR dynamic DMA window interface, but as a preliminary this patch
allows the user to reconfigure the window with new properties on the PHB
device.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>