qubes/storage: allow snapshots of snapshots

It may make sense to create 'snapshot' volume out of already 'snapshot',
not only 'origin'. In pracice it will exactly the same as 'snapshot
connected directly to 'origin'.

QubesOS/qubes-issues#866

qubes/storage/file.py

@@ -237,13 +237,27 @@ class FilePool(Pool):
             raise StoragePoolException("Unknown volume type " + volume_type)
 
         if volume_type in ['snapshot', 'read-only']:
-            origin_pool = vm.app.get_pool(
-                vm.template.volume_config[name]['pool'])
+            name = volume_config['name']
+
+            origin_vm = vm
+            while hasattr(origin_vm, 'template') and \
+                origin_vm.volume_config[name]['volume_type'] == \
+                    volume_type:
+                origin_vm = origin_vm.template
+
+            expected_origin_type = {
+                'snapshot': 'origin',
+                'read-only': 'read-write',  # FIXME: really?
+            }[volume_type]
+            assert origin_vm.volume_config[name]['volume_type'] == \
+                expected_origin_type
+
+            origin_pool = vm.app.get_pool(origin_vm.volume_config[name]['pool'])
             assert isinstance(origin_pool,
                               FilePool), 'Origin volume not a file volume'
-            volume_config['target_dir'] = origin_pool.target_dir(vm.template)
-            name = volume_config['name']
-            volume_config['size'] = vm.template.volume_config[name]['size']
+
+            volume_config['target_dir'] = origin_pool.target_dir(origin_vm)
+            volume_config['size'] = origin_vm.volume_config[name]['size']
         else:
             volume_config['target_dir'] = self.target_dir(vm)