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