Image show: print human readable string when the virtual size is unknown

Currently, when the virtual size of an image is not known, "None" is displayed.
To a regular user, it feels like a programming error. We try and make things
clearer by using a "human readable" string instead.

Change-Id: Id7b8799356857d9bc58cc8a3677024fe1a7f4f56
Partial-Bug: #1665037

glanceclient/common/utils.py

@@ -421,6 +421,8 @@ def print_image(image_obj, human_readable=False, max_col_width=None):
     ignore = ['self', 'access', 'file', 'schema']
     image = dict([item for item in image_obj.items()
                   if item[0] not in ignore])
+    if 'virtual_size' in image:
+        image['virtual_size'] = image.get('virtual_size') or 'Not available'
     if human_readable:
         image['size'] = make_size_human_readable(image['size'])
     if str(max_col_width).isdigit():