proxmox: add zfs module for retrieving importable zpool info

proxmox: add zfs module for retrieving importable zpool info

Will be used for prompting the user to rename existing "rpool" ZFS
pools, similar to what we do for an existing LVM "pve" volume group.

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>
Reviewed-by: Aaron Lauterer <a.lauterer@proxmox.com>
Tested-by: Aaron Lauterer <a.lauterer@proxmox.com>
 [ TL: added a bit context for what this will be used and merge in the
   tests into this preparatory commit ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>