o hotplug slots add
When the hot-added PCI device is p2p bridge, acpiphp calls
find_p2p_bridge() to add hotplug slots.
o hotplug slots remove
When the hot-removing PCI device is p2p bridge, acpiphp
calls cleanup_p2p_bridge() to remove hotplug slots.
o notify handler exchange
When the p2p bridge is added, acpiphp changes the notify
hanlder.
If no bridge device is inserted into the hotpluggable PCI
slot, acpiphp installs the notify handler for function.
After the p2p bridge hot-add, acpiphp has to install the
notify handler for bridge. Because, the role of the
handlers are not same. The hot-remove case is ditto.