trace: Fix 'char **' compilation error in simple backend

Currently, the generated function body will do "strlen(arg)" but the
argument could be 'char **' or 'char * const *'. Avoid that by excluding
such cases in is_string check.

Reported by patchew's "make docker-test-mingw@fedora".

Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1477453806-21097-1-git-send-email-famz@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>