lvmdbusd: Add a new 'Shared' property to the Vg interface

[vojtechtrefny]

@tasleson can you please look at this. We now have a request for a simple support for shared VGs for GFS2 so we might need this property in the future for libblockdev/blivet.

[tasleson]

@vojtechtrefny Thanks, committed to upstream and lvmteam repositories.