storage/file: create -cow.img only when needed

Do not create it at volume creation time. It it needed only when VM is
running, so create it just before startup only.

QubesOS/qubes-issues#2256

qubes/storage/file.py

@@ -164,9 +164,6 @@ class FileVolume(qubes.storage.Volume):
             'Volume size must be > 0'
         if not self.snap_on_start:
             create_sparse_file(self.path, self.size)
-        # path_cow not needed only in volatile volume
-        if self.save_on_stop or self.snap_on_start:
-            create_sparse_file(self.path_cow, self.size)
 
     def remove(self):
         if not self.snap_on_start:

qubes/tests/storage_file.py

@@ -406,8 +406,7 @@ class TC_03_FilePool(qubes.tests.QubesTestCase):
                                    expected_private_path)
 
         expected_rootcow_path = os.path.join(expected_vmdir, 'root-cow.img')
-        self.assertEqualAndExists(vm.volumes['root'].path_cow,
-                                   expected_rootcow_path)
+        self.assertEqual(vm.volumes['root'].path_cow, expected_rootcow_path)
 
     def assertEqualAndExists(self, result_path, expected_path):
         """ Check if the ``result_path``, matches ``expected_path`` and exists.