s390x: implement SIGP restart and shutdown

s390x: implement SIGP restart and shutdown

An s390x OS does reboot and shutdown triggers through hypercalls that
we didn't implement on the TCG backend yet. That means that so far we
couldn't shut down virtual machines for example, having them hang on
shutdown when not using KVM.

With this patch, this restriction is gone. We can now shut down and
reboot s390x virtual machines even when using the TCG backend.

Signed-off-by: Alexander Graf <agraf@suse.de>