s390x/tcg: don't exit the cpu loop in s390_cpu_virt_mem_rw()

s390_cpu_virt_mem_rw() must always return, so callers can react on
an exception (e.g. see ioinst_handle_stcrw()).

Therefore, using program_interrupt() is wrong. Fix that up.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171130162744.25442-9-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
