drivers: Add zfs device to allow use of zfs commands

Previously, zfs device was being only provided to allow the use of
commands needed to create the zpool, and so the file system.
At that time, doing so was quite enough, however, making zfs
device, i.e. /dev/zfs part of every OSv instance would allow us
to use commands that will help analysing, debugging, tuning
the zpool and file systems there contained.

The basic explanation is that those commands use libzfs which in
turn relies on /dev/zfs to communicate with the zfs code.

Commands example:
zpool, zfs, zdb. The latter one not being ported to OSv yet.
This patch will also be helpful for the ongoing ztest porting.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>