target/s390x: Improve heuristic for ipte

If only the page index is set, most likely we don't have a valid
virtual address. Let's do a full tlb flush for that case.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170622094151.28633-3-david@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
