LIO: untaint values read from remote config

LIO: untaint values read from remote config

The LIO backend for ZFS over iSCSI fetches the json-config periodically from
the target.
This patch reduces the stored config values to those which are actually used
and additonally untaints the values read from the remote host's config-file.

Since the LUN index is used in calls to targetcli on the remote host (via
run_command), untainting prevents the call to crash when run with '-T'.

Tested by creating a zfs over iscsi backed VM, starting it, adding disks,
resizing disks, removing disks, creating snapshots, rolling back to a snapshot.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>