core: clearing snapshot_id on disk detach

When detaching a DiskImage, the snapshot [1] ID should be cleared.
Otherwise, 'DiskImageDAO -> getAllSnapshotsForVmSnapshot' returns
floating DiskImages (which shouldn't be taken into account in any
flow since they're not relevant with regards to snapshots).

Note: when attaching a DiskImage, the snapshot ID is already updated
      properly according to the VM.

[1] only an active snapshot since detaching a disk with snapshots
    is blocked at canDo phase.

Change-Id: Iad60d2a6500f4bd738095949c7e36a4ec34122c3
Bug-Url: https://bugzilla.redhat.com/1015170
Signed-off-by: Daniel Erez <derez@redhat.com>