xics: XICS should not be a SysBusDevice

xics: XICS should not be a SysBusDevice

Currently xics - the component of the IBM POWER interrupt controller
representing the overall interrupt fabric / architecture is
represented as a descendent of SysBusDevice.  However, this is not
really correct - the xics presents nothing in MMIO space so it should
be an "unattached" device in the current QOM model.

Since this device will always be created by the machine type, not created
specifically from the command line, and because it has no migrated state
it should be safe to move it around the device composition tree.

Therefore this patch changes it to a descendent of TYPE_DEVICE, and
makes it an unattached device.  So that its reset handler still gets
called correctly, we add a qdev_set_parent_bus() to attach it to
sysbus.  It's not really clear that's correct (instead of using
register_reset()) but it appears to a common technique.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[clg corrected problems with reset]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
[dwg folded together and updated commit message]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>