nvme: move struct nvme_iod to pci.c

This structure is specific to the PCIe driver internals and should be moved
to pci.c.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>