Here's the first thing to try, depending on how damaged your system is.
Try booting into Single User Mode. That will give you a very simple root shell which will bypass nearly everything else. I can't guarantee that the stuff you'll need will work without libxml, but it's likely.
To do this, boot while holding down the Command and S key. It should boot into a shell. Follow the onscreen directions to fsck and remount the filesystem as writeable, and restore your backup copies.
Apple Examiner has an article with more details on the process.
If this doesn't work, you may need to find your install discs and see if you can get to a terminal from there.