Editing Xen DomU config file


I am trying to get my USB flash drive been accessed by the guest Windows Vista on Xen. I located the config file under /etc/xen/vm. However, the files are not accessible by vi or KATE. They simply stopped and gave me no further information. Am I locating the right files? Do I have to use certain account to edit them?

Also, there seems to be a plain text config file and a XML config file for the same VM. Which one should I work with?