Uses the ZFS utils to detect, import and mount zpools. These are
available as a new Bucket type 'zpool'.
Requires some minor changes to the existing disk and partiton detection
code, so the ZFS-specific part can use the information gathered in the
previous pass to associate drive names with their 'drive-xxxN.img.fidx'
node.
For detecting size, the zpool has to be imported. This is only done with
pools containing 5 or less disks, as anything else might take too long
(and should be seldomly found within VMs).
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>