mkfs: Switch to the newly created zfs device driver

Remove the embedded driver, and start using the one from drivers
sub-system. Still relies on the manual creation of /etc/mnttab as
the upload manifest wasn't yet processed.

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>