Xen error: xc_dom_find_loader

One of my Xen virtualization platforms crashed yesterday, throwing the error:

Error: (2, 'Invalid kernel', 'xc_dom_find_loader: no loader found\n')

Had the hardest time figuring out the cause.

Then, for whatever reason, I did a df -h and found my the /var/lib/xen mount was at 100%. I wiped out the contents of the save subdir, and moved all the boot_ files to /tmp. Suddenly, all my VMs could start.