storage/file: simplify search for origin volume of snapshot/read-only

Check directly vm.template, throwing AttributeError when not found.
There may be some value in converting it to more descriptive error, but
since that's mostly for internal users (not user facing actions) don't
bother for now.

QubesOS/qubes-issues#1842

qubes/storage/file.py

@@ -239,10 +239,8 @@ class FilePool(Pool):
         if volume_type in ['snapshot', 'read-only']:
             name = volume_config['name']
 
-            origin_vm = vm
-            while hasattr(origin_vm, 'template') and \
-                origin_vm.volume_config[name]['volume_type'] == \
-                    volume_type:
+            origin_vm = vm.template
+            while origin_vm.volume_config[name]['volume_type'] == volume_type:
                 origin_vm = origin_vm.template
 
             expected_origin_type = {