Get data from all block devices not just mounted

[loftyhauser]

When getting disk usage data, search through all block devices, not just those that are mounted. Thus usage data of partitions used as LVM or LUKS partitions are also included in the data graph. The user can use the filter option if there are block devices that are not desired to be in the graph. The code used to loop through the block devices is the same as that used in multiload_graph_disk_get_filter.

[nylen]

+1 for this approach. I ended up implementing something similar in https://github.com/nylen/multiload-ng/commit/e2e3b96a2a4bd27a5d20cb0aa50a767af37e889d.