Fix snapshot chain being deleted on XenServer (#9447)

Using XenServer as the hypervisor, when deleting a snapshot that has a parent, that parent will also get erased on storage, causing data loss. This behavior was introduced with #7873, where the list of snapshot states that can be deleted was changed to add BackedUp snapshots.

This PR changes the states list back to the original list, and swaps the while loop for a do while loop to account for the changes in #7873.

Fixes #9446

Author: JoaoJandre

engine/storage/snapshot/src/main/java/org/apache/cloudstack/storage/snapshot/DefaultSnapshotStrategy.java

@@ -102,6 +102,8 @@ public class DefaultSnapshotStrategy extends SnapshotStrategyBase {
     @Inject
     SnapshotZoneDao snapshotZoneDao;
 
+    private final List<Snapshot.State> snapshotStatesAbleToDeleteSnapshot = Arrays.asList(Snapshot.State.Destroying, Snapshot.State.Destroyed, Snapshot.State.Error);
+
     public SnapshotDataStoreVO getSnapshotImageStoreRef(long snapshotId, long zoneId) {
         List<SnapshotDataStoreVO> snaps = snapshotStoreDao.listReadyBySnapshot(snapshotId, DataStoreRole.Image);
         for (SnapshotDataStoreVO ref : snaps) {
@@ -199,9 +201,8 @@ protected boolean deleteSnapshotChain(SnapshotInfo snapshot, String storageToStr
 
         boolean result = false;
         boolean resultIsSet = false;
-        final List<Snapshot.State> snapshotStatesAbleToDeleteSnapshot = Arrays.asList(Snapshot.State.BackedUp, Snapshot.State.Destroying, Snapshot.State.Destroyed, Snapshot.State.Error);
         try {
-            while (snapshot != null && snapshotStatesAbleToDeleteSnapshot.contains(snapshot.getState())) {
+            do {
                 SnapshotInfo child = snapshot.getChild();
 
                 if (child != null) {
@@ -247,7 +248,7 @@ protected boolean deleteSnapshotChain(SnapshotInfo snapshot, String storageToStr
                 }
 
                 snapshot = parent;
-            }
+            } while (snapshot != null && snapshotStatesAbleToDeleteSnapshot.contains(snapshot.getState()));
         } catch (Exception e) {
             s_logger.error(String.format("Failed to delete snapshot [%s] on storage [%s] due to [%s].", snapshotTo, storageToString, e.getMessage()), e);
         }