Disks: instantiate import unit for created zpool

Disks: instantiate import unit for created zpool

When creating a new ZFS storage, also instantiate an import-unit for the pool.
This should help mitigate the case where some pools don't get imported during
boot, because they are not listed in an existing zpool.cache file.

This patch needs the corresponding addition of 'zfs-import@.service' in
the zfsonlinux repository.

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