spapr-pci: enable adding PHB via -device

Recent changes introduced cannot_instantiate_with_device_add_yet
and removed capability of adding yet another PCI host bridge via
command line for SPAPR platform (POWERPC64 server).

This brings the capability back and puts SPAPR PHB into "bridge"
category.

This is not much use for emulated PHB but it is absolutely required
for VFIO as we put an IOMMU group onto a separate PHB on SPAPR.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
tests: Add spapr-pci-host-bridge qtest

This adds a test whether sPAPR PHB can be added via the command line.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Andreas Färber <afaerber@suse.de>