core: prevent illegal disk on custom preview active snapshot

* SnapshotVmConfigurationHelper:
  avoid marking disks as illegal for a 'PREVIEW' snapshot
  as it represents previous 'Active VM' state.
* SnapshotsManager -> attempToRestoreVmConfigurationFromSnapshot:
  updating images according the specified list when VmConfiguration
  is not available - i.e. custom snapshotting to active VM
  (prevents a possible scenario of missing disks on active VM).

Change-Id: I1561107e05de155f9483e1f8a00d2dba3a4ab342
Bug-Url: https://bugzilla.redhat.com/1064067
Signed-off-by: Daniel Erez <derez@redhat.com>