Fix export locking

If qubesd has restarted then _export_lock will be None

qubes/storage/file.py

@@ -183,7 +183,7 @@ def __init__(self, dir_path, **kwargs):
         self.dir_path = dir_path
         assert self.dir_path, "dir_path not specified"
         self._revisions_to_keep = 0
-        self._locked = None
+        self._export_lock = None
         super().__init__(**kwargs)
 
         if self.snap_on_start:
@@ -268,17 +268,17 @@ def commit(self):
         return self
 
     def export(self):
-        if self._locked is not None:
-            assert self._locked is FileVolume._marker_running, \
+        if self._export_lock is not None:
+            assert self._export_lock is FileVolume._marker_running, \
                 'nested calls to export()'
             self._not_implemented('exporting a dirty volume')
-        self._locked = FileVolume._marker_exported
+        self._export_lock = FileVolume._marker_exported
         return self.path
 
     def export_end(self, path):
-        assert self._locked is FileVolume._marker_exported, \
-            'cannot end an export that never began'
-        self._locked = None
+        assert self._export_lock is not FileVolume._marker_running, \
+            'ending an export on a running volume?'
+        self._export_lock = None
 
     @asyncio.coroutine
     def import_volume(self, src_volume):
@@ -322,13 +322,13 @@ def reset(self):
         return self
 
     def start(self):
-        if self._locked is not None:
-            assert self._locked is FileVolume._marker_exported, \
+        if self._export_lock is not None:
+            assert self._export_lock is FileVolume._marker_exported, \
                 'nested calls to start()'
             self._not_implemented('starting a VM with an exported volume')
         if self.is_dirty():
             self._not_implemented('exporting a dirty volume')
-        self._locked = FileVolume._marker_running
+        self._export_lock = FileVolume._marker_running
         if not self.save_on_stop and not self.snap_on_start:
             self.reset()
         else:
@@ -346,15 +346,15 @@ def start(self):
         return self
 
     def stop(self):
-        assert self._locked is FileVolume._marker_running, \
-            'cannot stop a volume that has not been started'
+        assert self._export_lock is not FileVolume._marker_exported, \
+            'trying to stop exported file volume?'
         if self.save_on_stop:
             self.commit()
         elif self.snap_on_start:
             _remove_if_exists(self.path_cow)
         else:
             _remove_if_exists(self.path)
-        self._locked = None
+        self._export_lock = None
         return self
 
     @property