Add try and except for template disks

It turns out that some templates don't need a disk size if they use a
image.

opennebula_api/serializers.py

@@ -50,9 +50,12 @@ class VirtualMachineTemplateSerializer(serializers.Serializer):
     def get_disk_size(self, obj):
         template = obj.template
         disk_size = 0
+        try:
             for disk in template.disks:
                 disk_size += int(disk.size)
             return disk_size / 1024 
+        except:
+            return 0
 
     def get_price(self, obj):
         template = obj.template