core: filter shareable disks on preview snapshot

TryBackToAllSnapshotsOfVm -> getImagesToPreview:
filter out shareable/nonsnapable disks.

Change-Id: I232d3387e203933bdbb12a5e3401c096278a41d3
Bug-Url: https://bugzilla.redhat.com/1161916
Signed-off-by: Daniel Erez <derez@redhat.com>