Petitecloud bug fix: unable to find install media

[Aryeh Friedman]

We have released 0.1.8 to fix a bug that made guest instances
uninstallable.   0.1.8 also contains a new -- but half done -- feature that
will be fully done in 0.1.9.   For now, ignore the "Start at boot" column
(unless you feel like hacking /usr/local/etc/petitcloud/instances).

It should also be noted that the file /usr/local/etc/petitcloud/instances
has a new format as of 0.1.8, making it incompatible with 0.1.7.  (Note
that we intend to freeze the file format sometime before 0.2.0 is released.)