Vagrantfile: fix typo; add Xdebug example

This change adds a commented-out configuration directive to forward port 9000.
This is meant to facilitate the configuration of remote debugging via Xdebug.

The change also corrects the Vagrantfile-extra.rb file name in the comment
block.

Change-Id: I31184864b1ac793f96f15cb1dbdfb39424835e60