Add another CentOS5 workaround

On this ancient distro the "mount -L" used by ONE's vminit doesn't
discover the volume unless it's already in blkidtab.

While ONE checks for the symlink /dev/disk/by-label/CONTEXT symlink
first it then resorts to "mount -L". So make sure we have an entry in
blkid.tab when contextualization starts.