nvme: move AER handling to common code

The transport driver still needs to do the actual submission, but all the
higher level code can be shared.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@fb.com>