hw/pci: reserve IO and mem for pci express downstream ports with no devices attached

Commit c6e298e1f12e0f4ca02b6da5e42919ae055f6830
    hw/pci: reserve IO and mem for pci-2-pci bridges with no devices attached

introduced support for hot-plugging devices behind pci-2-pci bridges.
Extend hotplug support also for pci express downstream ports.

Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>