zfs: create dataset var-lib-vz for /var/lib/vz

zfs: create dataset var-lib-vz for /var/lib/vz

Creating rpool/var/lib/vz and all intermediate datasets causes a
service-failure of `var.mount` upon shutdown.

creating the dataset for /var/lib/vz directly at the rpool and setting
its mountpoint property seems the most robust way to address this.

The alternative approach of setting `canmount=off` on the `var`
dataset seems a bit dangerous (users setting a zfs property and
suddenly hiding their /var contents).

The only small downside to this approach is that the setting of the
mountpoint happens quite a bit after extracting the data - but this
would probably be better addressed with a refactoring of the
lowlevel-installer code (setting the zfs-pool up under /target and
getting rid of a few special cases)

Fixes: dd19d40ceac179ba18652f1d6c3e4c23f246af00
Suggested-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>