fw/pci: do not automatically allocate IO region for PCIe bridges

fw/pci: do not automatically allocate IO region for PCIe bridges

PCIe downstream ports (Root Ports and switches Downstream Ports) appear
to firmware as PCI-PCI bridges and a 4K IO space is allocated for them
even if there is no device behind them requesting IO space,
all that for hotplug purpose.

However, PCIe devices can work without IO, so there is no need
to allocate IO space for hotplug.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>