bug 6892: update resourceCount for snapshot after the DB record is created; if snapshot fails to create on the backend - reduce resource count

Also fixed couple of other issues:
* usage event generation - generate event only when snapshot is created on primary and backed up on secondary
* zoneId was always set to 0 for snapshot.delete event, fixed this.
* Fixed resource_count decrement for manual snapshot deletion

api/src/com/cloud/api/commands/CreateSnapshotCmd.java

@@ -119,7 +119,7 @@ public class CreateSnapshotCmd extends BaseAsyncCreateCmd {
     }
     
     @Override
-    public void create(){
+    public void create() throws ResourceAllocationException{
         Snapshot snapshot = _snapshotService.allocSnapshot(this);
         if (snapshot != null) {
             this.setEntityId(snapshot.getId());
@@ -129,7 +129,7 @@ public class CreateSnapshotCmd extends BaseAsyncCreateCmd {
     }
     
     @Override
-    public void execute() throws ResourceAllocationException{ 	
+    public void execute() { 	
         Snapshot snapshot = _snapshotService.createSnapshot(this);
         if (snapshot != null) {
             SnapshotResponse response = _responseGenerator.createSnapshotResponse(snapshot);

api/src/com/cloud/storage/snapshot/SnapshotService.java

@@ -37,7 +37,7 @@ public interface SnapshotService {
      * @param cmd the API command wrapping the parameters for creating the snapshot (mainly volumeId) 
      * @return the Snapshot that was created
      */
-    Snapshot createSnapshot(CreateSnapshotCmd cmd) throws InvalidParameterValueException, ResourceAllocationException;
+    Snapshot createSnapshot(CreateSnapshotCmd cmd) throws InvalidParameterValueException;
 
     /**
      * List all snapshots of a disk volume. Optionally lists snapshots created by specified interval
@@ -79,6 +79,6 @@ public interface SnapshotService {
     
     boolean deleteSnapshotPolicies(DeleteSnapshotPoliciesCmd cmd);
     
-    Snapshot allocSnapshot(CreateSnapshotCmd cmd);
+    Snapshot allocSnapshot(CreateSnapshotCmd cmd) throws ResourceAllocationException;
 
 }

server/src/com/cloud/storage/snapshot/SnapshotManager.java

@@ -44,7 +44,7 @@ public interface SnapshotManager {
      * @param cmd the API command wrapping the parameters for creating the snapshot (mainly volumeId) 
      * @return the Snapshot that was created
      */
-    SnapshotVO createSnapshotImpl(Long volumeId, Long policyId, Long snapshotId) throws ResourceAllocationException;
+    SnapshotVO createSnapshotImpl(Long volumeId, Long policyId, Long snapshotId);
 
     /**
      * After successfully creating a snapshot of a volume, copy the snapshot to the secondary storage for