ppc/pnv: populate device tree for IPMI BT devices

When an ipmi-bt device [1] is defined on the ISA bus, we need to
populate the device tree with the object properties. Such devices are
created with the command line options :

   -device ipmi-bmc-sim,id=bmc0 -device isa-ipmi-bt,bmc=bmc0,irq=10

[1] https://lists.gnu.org/archive/html/qemu-devel/2015-11/msg03168.html



Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>