core: Fix snapshot quota when removing a VM

When VM is removed the disks quota is released.
Added a release for the snapshot's quota as well

Change-Id: I5a1649e65c1a0cf4c7752dd5f6e39c9b034feecd
Bug-Url: https://bugzilla.redhat.com/1001017
Signed-off-by: Ofri Masad <omasad@redhat.com>