gennvm: Generic NVM manager

The implementation for Open-Channel SSDs is divided into media
management and targets. This patch implements a generic media manager
for open-channel SSDs. After a media manager has been initialized,
single or multiple targets can be instantiated with the media managed as
the backend.

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