KVM: PPC: Book3S HV: Fix preempted vcore list locking

When a vcore gets preempted, we put it on the preempted vcore list for
the current CPU.  The runner task then calls schedule() and comes back
some time later and takes itself off the list.  We need to be careful
to lock the list that it was put onto, which may not be the list for the
current CPU since the runner task may have moved to another CPU.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>