Use Nvlist keys from ZFS Header
For accessing data in zfs nvlists, we declare strings on the go and use them for fetching different data from the nvlist. However this is not ideal and we should be using consts defined in zfs header as this will reduce chances of error where we mistakingly change a key's name and also help increase readability and improve our py-libzfs code base in general
Use constants defined in zfs header
This commit introduces changes to most of the strings which have been defined on the go and could be imported from zfs header. We import them from zfs header now to make the code base more consistent and also avoid potential errors in the future.