Simplify the arguments to nvme_transport_ctrlr_scan to take
a transport id that identifies the discovery service (or
NULL to scan PCIe).
Further, separate scan into two functions - scan and attach.
Scan is for scanning an entire bus, attach is for a specific
device.
Change-Id: I464f351a02a04bc5a45096dcf5dc8fc5ac489041
Signed-off-by: Ben Walker <benjamin.walker@intel.com>