spapr: Ensure CPU cores are added contiguously and removed in LIFO order

If CPU core addition or removal is allowed in random order leading to
holes in the core id range (and hence in the cpu_index range), migration
can fail as migration with holes in cpu_index range isn't yet handled
correctly.

Prevent this situation by enforcing the addition in contiguous order
and removal in LIFO order so that we never end up with holes in
cpu_index range.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>