core: add foreign key to quota_id

problem: quota_id field (in vm_static and images) isn't linked
to quota table, therefore when removing a quota, it doesn't clear
the quota_id ref.
solution: set quota_id fields as foreign keys and use
set NULL on delete.
Currently there is extensive usage of Guid.Empty to indicate
null, removed that in relevant DAOs.
Upgrade:
- set NULL instead of Guid.Empty
- set NULL instead of non-existing quota ids
- add the foreign key constraint

Change-Id: I5092a3070937719009fa1fb4fb9ecb792f03b969
Bug-Url: https://bugzilla.redhat.com/1003839
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>