target/s390x: factor out handling of WAIT PSW into s390_handle_wait()

This will now also detect crashes under TCG. We can directly use
cpu->env.psw.addr instead of kvm_run, as we do a cpu_synchronize_state().

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170928203708.9376-9-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
