s390x: handle exceptions during s390_cpu_virt_mem_rw() correctly (TCG)

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

However, for TCG we always have to exit the cpu loop (and restore the
cpu state before that) if we injected a program interrupt. So let's
introduce and use s390_cpu_virt_mem_handle_exc() in code that is not
purely KVM.

Directly pass the retaddr we already have available in these functions.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171130162744.25442-8-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
