spapr_pci: Introduce a finish_realize() callback

The spapr-pci PHB initializes IOMMU for emulated devices only.
The upcoming VFIO support will do it different. However both emulated
and VFIO PHB types share most of the initialization code.
For the type specific things a new finish_realize() callback is
introduced.

This introduces sPAPRPHBClass derived from PCIHostBridgeClass and
adds the callback pointer.

This implements finish_realize() for emulated devices.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
[agraf: Fix compilation]
Signed-off-by: Alexander Graf <agraf@suse.de>
