core: fix unchecked cast

Coverity scan detected unchecked cast from VdcActionParametersBase to
ImagesActionsParametersBase in CreateAllSnapshotsFromVmCommand class.
This patch solved it by verifing with 'instanceof' check that the cast
is safe.

Change-Id: I7da9f46ae0742efb376ce558be48b9357e906e85
Signed-off-by: Arik Hadas <ahadas@redhat.com>