hw/pci-bridge: format special OFW unit address for PXB host

We have agreed that OpenFirmware device paths in the "bootorder" fw_cfg
file should follow the pattern

  /pci@i0cf8,%x/...

for devices that live behind an extra root bus. The extra root bus in
question is the %x'th among the extra root buses. (In other words, %x
gives the position of the affected extra root bus relative to the other
extra root buses, in bus_nr order.) %x starts at 1, and is formatted in
hex.

The portion of the unit address that comes before the comma is dynamically
taken from the main host bridge, similarly to sysbus_get_fw_dev_path().

Cc: Kevin O'Connor <kevin@koconnor.net>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>