hw/pcie-root-port: Fix hotplug for PCI devices requiring IO

hw/pcie-root-port: Fix hotplug for PCI devices requiring IO

Q35 has now ACPI hotplug enabled by default for PCI(e) devices.
As opposed to native PCIe hotplug, guests like Fedora 34
will not assign IO range to pcie-root-ports not supporting
native hotplug, resulting into a regression.

Reproduce by:
    qemu-bin -M q35 -device pcie-root-port,id=p1 -monitor stdio
    device_add e1000,bus=p1
In the Guest OS the respective pcie-root-port will have the IO range
disabled.

Fix it by setting the "reserve-io" hint capability of the
pcie-root-ports so the firmware will allocate the IO range instead.

Acked-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Message-Id: <20210802090057.1709775-1-marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>