For those that didn't know, KVM hangs bad when an NFS mount disappears. I ran into an issue where I forgot to delete an old NFS data share. The current one was up, but I disabled the old node. Well that went badly. Eventually I figured out that other than fstab, I needed to remove the xml file and a symlink to it -- etc/libvirt/storage (the xml file) and /etc/libvirt/storage/autostart (symlink).