nvme-print-stdout: Support subsys reachable from several hosts

The host might be connected with several different hostnqn, and
this might result that the same subsystem is visible from several
hosts.

The current code assumes a each subsystem shows up only once. So
we need to iterate over all hosts/subsystem tuples and print
them accordingly.

Use hash tables to avoid triple loop iterations. Also using strset sorts
all the names alphabetically which is also nice to have.

Signed-off-by: Daniel Wagner <[email protected]>