Disable shrinking QCOW2 volumes

Since QCOW2 volumes cannot be resized to lower value in any case,
just disable it in backend.

Author: ravening

server/src/main/java/com/cloud/storage/VolumeApiServiceImpl.java

@@ -1054,6 +1054,14 @@ public VolumeVO resizeVolume(ResizeVolumeCmd cmd) throws ResourceAllocationExcep
              * This will be checked again at the hypervisor level where we can see
              * the actual disk size.
              */
+            if (currentSize > newSize) {
+                VolumeVO vol = _volsDao.findById(cmd.getEntityId());
+                if (vol != null && ImageFormat.QCOW2.equals(vol.getFormat()) && !Volume.State.Allocated.equals(volume.getState())) {
+                    String message = "Unable to shrink volumes of type QCOW2";
+                    s_logger.warn(message);
+                    throw new InvalidParameterValueException(message);
+                }
+            }
             if (currentSize > newSize && !shrinkOk) {
                 throw new InvalidParameterValueException("Going from existing size of " + currentSize + " to size of " + newSize + " would shrink the volume."
                         + "Need to sign off by supplying the shrinkok parameter with value of true.");