nvme-discover: lookup existing persistent controllers

nvme-discover: lookup existing persistent controllers

If persistent controller connections are present they should be
preferred even if no --device option is given on the commandline.

To avoid selecting a temporary non-persistent controller the
'kato' attribute is checked; any controller for which the
attribute is '0' will be skipped. This logic is not applied on
older kernels that don't support the 'kato' attribute. On these
kernels, we just have to assume that the encountered discovery
controller is persistent.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin Wilck <mwilck@suse.de>