cyclades: Fix corner case when updating vm disks

Fix a corner case when updating the disks of an instance, in
snf-dispatcher and reconciliation. This corner case is when a VM has both:

* unknown Ganeti instances (without a unique Synnefo ID)
* volumes that exist in DB but not in Ganeti

Such a case cannot be fixed automatically, and should be resolved by the
administrator. This case will also appear during the upgrade to version
0.16.