test-cutils: Add missing qemu_strtosz()... endptr checks

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <1487708048-2131-11-git-send-email-armbru@redhat.com>
