CLOUDSTACK-7301, CLOUDSTACK-7947: double check if parent snapshot is removed or not

when creating new snapshot, check if parent snapshot is removed or not

Reviewed-by: Min
(cherry picked from commit bd799653293f9dc257ab68a3b04f6ea769e08e36)
Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>

Conflicts:
	engine/storage/snapshot/src/org/apache/cloudstack/storage/snapshot/XenserverSnapshotStrategy.java