core: organize VDSReturnValue class

1. Move all fields declarations up to beginning of the class
2. Remove the '_' prefix from fields names
3. Move the constructor to be right after the fields declarations
4. Remove the 'private' prefix from field names

Change-Id: I7a5ceaa2271dc934577a55f7de596cbcef7e25cd
Signed-off-by: Arik Hadas <ahadas@redhat.com>