s390x/tcg: fix DIAG 308 with > 1 VCPU (MTTCG)

Currently, multi threaded TCG with > 1 VCPU gets stuck during IPL, when
the bios tries to switch to the loaded kernel via DIAG 308.

As run_on_cpu() is used, we run into a deadlock after handling the reset.
We need the iolock (just like KVM).

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171116170526.12643-4-david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
