I was able to accomplish what I wanted (remove the port from the url and make it
accessible from the root path of the domain. Here's what I did (on Ubuntu linux):
install apache: sudo apt-get install apache2-mpm-prefork
enable some apache modules: sudo a2enmod proxy_http
Create a virtual host file in /etc/apache2/sites-available with the following contents:
<VirtualHost *:80>
    ServerName 
wiki.yourdomain.com
    DocumentRoot /var/www
    <Directory "/var/www/">
        Options -Indexes Includes FollowSymLinks
        AllowOverride All
        Allow from all
    </Directory>
    RedirectMatch permanent ^/$ 
http://wiki.yourdomain.com/xwiki/
    ProxyPass /xwiki 
http://localhost:8080/xwiki
    ProxyPassReverse /xwiki 
http://localhost:8080/xwiki
</VirtualHost>
        4. restart apache: sudo service apache2 restart