storeage/reflink: unlock size getter

Don't update _size in the getter, so it can be unlocked (which is
helpful for QubesOS/qubes-issues#5935).

!!! If cherry-picking for release4.0, also adjust import_data() to !!!
!!! use self.size (no underscore) instead of self._get_size()      !!!

(cherry picked from commit a1b5262)

qubes/storage/reflink.py

@@ -217,7 +217,7 @@ def start(self):
                 # Preferably use the size of a leftover image, in case
                 # the volume was previously resized - but then a crash
                 # prevented qubes.xml serialization of the new size.
-                _create_sparse_file(self._path_dirty, self._get_size())
+                _create_sparse_file(self._path_dirty, self.size)
         return self
 
     @_coroutinized
@@ -227,7 +227,7 @@ def stop(self):
             self._commit(self._path_dirty)
         else:
             if not self.snap_on_start:
-                self._get_size()  # preserve manual resize of image
+                self._size = self.size  # preserve manual resize of image
             _remove_file(self._path_dirty)
             _remove_file(self._path_clean)
         return self
@@ -298,7 +298,7 @@ def import_data(self):
         if not self.save_on_stop:
             raise NotImplementedError(
                 'Cannot import_data: {} is not save_on_stop'.format(self.vid))
-        _create_sparse_file(self._path_import, self._get_size())
+        _create_sparse_file(self._path_import, self.size)
         return self._path_import
 
     def _import_data_end(self, success):
@@ -339,15 +339,13 @@ def revisions(self):
         return collections.OrderedDict(sorted(items,
                                               key=lambda item: int(item[0])))
 
-    def _get_size(self):
+    @property
+    def size(self):
         for path in (self._path_dirty, self._path_clean):
             with suppress(FileNotFoundError):
-                self._size = os.path.getsize(path)
-                break
+                return os.path.getsize(path)
         return self._size
 
-    size = property(_locked(_get_size))
-
     @property
     def usage(self):
         ''' Return volume disk usage from the VM's perspective. It is