[pve-devel] pve-storage : add zfs storage plugin

[Alexandre Derumier]

This is the zfs storage plugin from Michael + my cleanup

Tested on omnios and nexenta (community and enterprise).

(I'm think about to drop the nexenta plugin, as it's working 100% with this plugin, 
 and it's a lot faster than nexenta api + solve the missing zfs rename command from nexenta api)



Note for Michael : 

I have done 2 last cleanup since our discussion:

1 on sub zfs_get_pool_stats, calling only 1 ssh command to retrieve available and used space (in bytes)

1 on zfs_get_zvol_size, retrieve value in bytes directly