This adds support for a standard pci to pci bridge,
enabling support for more than 32 PCI devices in the system.
Device hotplug is supported by means of SHPC controller.
For guests with an SHPC driver, this allows robust hotplug
and even hotplug of nested bridges, up to 31 devices
per bridge.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>