Split pcie_cap_slot_hotplug() into hotplug/unplug callbacks
and register them as "hotplug-handler" interface implementation of
PCIE_SLOT device.

Replace pci_bus_hotplug() wiring with setting link on PCI BUS
"hotplug-handler" property to PCI_BRIDGE_DEV device.
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>