spapr_pci: map the MSI window in each PHB

On sPAPR, virtio devices are connected to the PCI bus and use MSI-X.
Commit cc943c36faa192cd4b32af8fe5edb31894017d35 has modified MSI-X
so that writes are made using the bus master address space and follow
the IOMMU path.

Unfortunately, the IOMMU address space address space does not have an
MSI window: the notification is silently dropped in unassigned_mem_write
instead of reaching the guest... The most visible effect is that all
virtio devices are non-functional on sPAPR since then. :(

This patch does the following:
1) map the MSI window into the IOMMU address space for each PHB
   - since each PHB instantiates its own IOMMU address space, we
     can safely map the window at a fixed address (SPAPR_PCI_MSI_WINDOW)
   - no real need to keep the MSI window setup in a separate function,
     the spapr_pci_msi_init() code moves to spapr_phb_realize().

2) kill the global MSI window as it is not needed in the end

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>