lightnvm: introduce ioctl to initialize device

Based on the previous patch, we now introduce an ioctl to initialize the
device using nvm_init_sysblock and create the necessary system blocks.
The user may specify the media manager that they wish to instantiate on
top. Default from user-space will be "gennvm".

Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>