s390x/kvm: factor out build_channel_report_mcic() into cpu.h

We'll need it later on in two places. Refactor it to just indicate the
validity bits. While at it, introduce a define for the used CR14 bit (we'll
also need later on).

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171208160207.26494-2-david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
