Allow parallel execution of backup and delete commands

Change executeInSequence() to return false for TakeBackupCommand
and DeleteBackupCommand, allowing the KVM agent to process multiple
backup/delete operations concurrently via its worker thread pool.

Previously, all backup commands were serialized — a large VM backup
(e.g. 100+ GB taking 2+ hours) would block all other backup and
delete operations on the same host. Since each backup mounts its
own temporary NFS directory and operates on independent VM disks,
there is no shared state requiring serialization.

Restore and PrepareForBackupRestoration commands remain sequential
as they modify VM state that should not be concurrent.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jmsperu

core/src/main/java/org/apache/cloudstack/backup/DeleteBackupCommand.java

@@ -71,6 +71,6 @@ public void setMountOptions(String mountOptions) {
 
     @Override
     public boolean executeInSequence() {
-        return true;
+        return false;
     }
 }

core/src/main/java/org/apache/cloudstack/backup/TakeBackupCommand.java

@@ -89,6 +89,6 @@ public void setVolumePaths(List<String> volumePaths) {
 
     @Override
     public boolean executeInSequence() {
-        return true;
+        return false;
     }
 }