Bug 17094: Make Koha::Virtualshelf methods return Koha::Objects-based objects

Bug 17094: Make Koha::Virtualshelf methods return Koha::Objects-based objects

Instead of DBIx::Class objects.

Test plan:
1/ Add content to a list and share it with another patron
2/ Try to view the list with the other patron
3/ download and send a shelf and check if the biblio list is correct
4/ prove t/db_dependent/Virtualshelves.t should return green

Signed-off-by: Aleisha Amohia <aleishaamohia@hotmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>