s390x: fix smp support for kvm

Currently smp support for kvm does not work. Qemu does a kvm run even on
secondary CPUs which dont have a sane state (initial psw == 0)
triggering some program faults. Architecturally these cpus are in the stopped
state, so we should not do the kvm run ioctl. (these CPUs will be started
by a SIGP restart later during the boot process)

We need to tell the loop that this cpu should not run. Jan Kiszka pointed
out that kvm_arch_process_async_events is the right place to do.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
