ppc/pnv: restrict BMC object to the BMC simulator

ppc/pnv: restrict BMC object to the BMC simulator

Today, when a PowerNV guest runs, it uses the sensor definitions of
the BMC simulator to populate the device tree. But an external IPMI
BMC could also be used and, in that case, it is not (yet) possible to
retrieve the sensor list. Generating the OEM SEL event for shutdown or
reboot also does not make sense as it should be generated on the BMC
side.

This change allows a guest to use an 'ipmi-bmc-extern' backend to the
'isa-ipmi-bt' device and a 'chardev' for transport such as :

	-chardev socket,id=ipmi0,host=localhost,port=9002,reconnect=10 \
	-device ipmi-bmc-extern,id=bmc0,chardev=ipmi0 \
	-device isa-ipmi-bt,bmc=bmc0,irq=10

and connect to a BMC simulator, the OpenIPMI ipmi_sim simulator for
instance.

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