Design correct reporting of storage free space

This is a long standing bug in Ganeti. Add a small design on how we plan
to fix this for Ganeti 2.7.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>