@ hanpedro -
LAMPP is not a 'standard LAMP stack'. its hard-wired to tbe bash file that makes everything work - /opt/lampp/lampp. On the windows version of XAMPP there are batch files that will basically re-configure all the options required to make XAMMP work in a different folder. As far as I know - that just does not exist in the Linux version. I suspect the reason is underlying file permissions and owenership issues, but I didn't write the script either. I suggest you open the file with an editor - you will be amazed at what you can learn. 'lampp' is going to need to stay where it is.
You certainly COULD move your Document Root to /var/www/html. You would want to set the same ownership and permissions that /opt/lampp/htdocs currently has.
as for the /opt/lamp/htdocs/xampp folder, you might want to try using an Alias (or SymLink) - I don't know if any of its parts are hard-wired, you would have to open the files and take a look. You could just copy it to the new location and see what happens - if it doesn't work then go with an Alias directive.