spapr_cpu_core: drop reference on ICP object during CPU realization

When a piece of code allocates an object, it implicitely gets a reference
on it. If it then makes that object a child property of another object, it
should drop its own reference at some point otherwise the child object can
never be finalized. The current code hence leaks one ICP object per CPU
when hot-removing a core.

Failing to add a newly allocated ICP object to the CPU is a bug. While here,
let's ensure QEMU aborts if this ever happens.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>