image-volume opened too many times when image being importing which is unnecessary

image-volume opened too many times when image being importing which is unnecessary.

```
# glance_store/_drivers/cinder.py #Store.add()

    def add(self, image_id, image_file, image_size, hashing_algo, context=None,
            verifier=None):
...
```

During the import image process, the 'image_size' passed to the add () method is always 0. According to the current logic, when the image is relatively large, the image-volume will be mounted/unmounted frequently because of the dynamic expansion of the mirror volume (for example, image with size 70G need to be mounted/unmounted 70/70 times)

I think when calling the add () method, we can get the actual size of 'stage_image'' in advance and pass it as' image_size''.