ppc/pnv: Create BMC devices at machine init

The BMC of the OpenPOWER systems monitors the machine state using
sensors, controls the power and controls the access to the PNOR flash
device containing the firmware image required to boot the host.

QEMU models the power cycle process, access to the sensors and access
to the PNOR device. But, for these features to be available, the QEMU
PowerNV machine needs two extras devices on the command line, an IPMI
BT device for communication and a BMC backend device:

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

The BMC properties are then defined accordingly in the device tree and
OPAL self adapts. If a BMC device and an IPMI BT device are not
available, OPAL does not try to communicate with the BMC in any
manner. This is not how real systems behave.

To be closer to the default behavior, create an IPMI BMC simulator
device and an IPMI BT device at machine initialization time. We loose
the ability to define an external BMC device but there are benefits:

  - a better match with real systems,
  - a better test coverage of the OPAL code,
  - system powerdown and reset commands that work,
  - a QEMU device tree compliant with the specifications (*).

(*) Still needs a MBOX device.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20191121162340.11049-1-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

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