libmultipath: allow dumping only "local" hwtable in snprint_config

libmultipath: allow dumping only "local" hwtable in snprint_config

This change is key for allowing to dump the "local" multipath configuration
only, i.e. those entries of the hardware table that match existing devices
in the system. By passing the output of the new helper function
get_used_hwes() as the last argument for snprint_config(), only the local
entries will be dumped.

The rationale is that "multipath -t" output is overwhelmingly long for most
use cases; in particular it's not useful as a template for creating a local
configuration.

Signed-off-by: Martin Wilck <mwilck@suse.com>