spapr: Allow hot plug/unplug of PCI bridges and devices under PCI bridges

spapr: Allow hot plug/unplug of PCI bridges and devices under PCI bridges

The pseries machine type already allows PCI hotplug and unplug via the
PAPR mechanism, but only on the root bus of each PHB.  This patch extends
this to allow PCI to PCI bridges to be hotplugged, and devices to be
hotplugged or unplugged under P2P bridges.

For now we disallow hot unplugging P2P bridges.  I tried doing that, but
haven't managed to get it working, I think due to some guest side problems
that need further investigation.

To do this we dynamically construct DRCs when bridges are hot (or cold)
added, which can in turn be used to hotplug devices under the bridge.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>