config: fix metadata/disk_areas config setting registration

config: fix metadata/disk_areas config setting registration

The metadata/disk_areas setting was incorrectly registered as
"string" configuration option but it's a section where each area
is defined in its own subsection with "start_sector", "size" and "id"
setting.

This setting is not officialy supported, it's undocumented and it's
used solely for debugging.

Note: At this moment, it does not seem to be working with lvmetad!