Make cluster-verify check the drbd minors space

This patch adds support for verification of drbd minors space in cluster
verify: minors which belong to running instances and should be online
but are not, and minors which do not belong to any instace but are in
use.

The patch requires exposing some methods from bdev.DRBD8 and
config.ConfigWriter which were until now private methods.

Reviewed-by: ultrotter