FreeNAS Code

make the disk replacement works if there is more than one disk unavail

introduce a Pool object while keeping the legacy API for now

make the zpool status parser deal with UNAVAIL

significant rewrite of zpool status parser while keeping the same API
work is still required for a new pythonic API

move some classes/method related to zfs for future use in replacement;
a lot of work is still required for a better API for a more pythonic
way of retrieving the state of the pool