core: block preview of active vm snapshot

* TryBackToAllSnapshotsOfVmCommand -> canDo:
- Active VM snapshot validation.

* DiskSnapshotsValidator:
- Added canDiskSnapshotsBePreviewed validation.
- Allow custom preview of active VM configuration only
  when selecting a subset of snapshot disks.

Change-Id: Iee5d5e6716cec69f36e2e98fa2c1578c53dc2384
Bug-Url: https://bugzilla.redhat.com/1096884
Signed-off-by: Daniel Erez <derez@redhat.com>