Add two helpers: one to lookup for a given IOTLB entry and
one to insert a new entry. We also move the tracing there.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200728150815.11446-3-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>