change logger level from debug to error

Author: Gean Jair Silva

plugins/hypervisors/kvm/src/main/java/com/cloud/hypervisor/kvm/storage/LibvirtStorageAdaptor.java

@@ -677,7 +677,7 @@ private Long getBackingFileSizes(StoragePool pool, StorageVol vol) throws Libvir
             try {
                 current = getVolume(pool, backingName);
             } catch (CloudRuntimeException e) {
-                logger.debug("Unable to resolve backing volume {} in pool {}: {}", backingName, pool.getName(), e.getMessage());
+                logger.error("Unable to resolve backing volume {} in pool {}: {}", backingName, pool.getName(), e.getMessage());
                 break;
             }
         }