hw/acpi: fix a DSDT table issue when a pxb is present.

PXBs do not support hotplug so they don't have a PCNT function.
Since the PXB's PCI root-bus is a child bus of bus 0, the
build_dsdt code will add a call to the corresponding PCNT function.

Fix this by skipping the PCNT call for the above case.
While at it skip also PCIe child buses.

Reported-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>