zfs: automatically import extra pools

This patch will provide OSv with an important functionality when
multiple ZFS pools are being used in the system. It follows a ZFS on
Linux scheme where all available pools in the system are imported and
their dataset mounted during system initialization process.

It was tested both locally and on EC2.

Details on EC2 test: Started a EC2 instance, created a new ZFS pool
comprising two additional non-ephemeral disks, stopped the instance,
re-started the instance, meaning a new OSv instance, and checked that
the new ZFS pool would be automatically available on subsequent OSv
instances.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>