pseries: Abolish envs array

Currently the pseries machine init code builds up an array, envs, of
CPUState pointers for all the cpus in the system.  This is kind of
pointless, given the generic code already has a perfectly good linked list
of the cpus.

In addition, there are a number of places which assume that the cpu's
cpu_index field is equal to its index in this array.  This is true in
practice, because cpu_index values are just assigned sequentially, but
it's conceptually incorrect and may not always be true.

Therefore, this patch abolishes the envs array, and explicitly uses the
generic cpu linked list and cpu_index values throughout.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>