mpathconf: fix read_blacklist use of logging library

[aesteve-rh]

Currently, read_blacklist is using logging to log warnings to the console. But the function is called from the tool.config_lvm_filter module, so the logs will be hidden when using vdsm-tool.

We can fix it now and return some additional information that can be processed by the config_lvm_filter module and print the correct logs into sys.stdout.

Note that there is #258, which could allow passing the right logging context through a parameter as an alternate solution.

[nirs]

Passing a logger sounds like the right approach.