xcp/accessor.py: Fix static analyis warnings for self.location

Fix static analysis warnings in `MountingAccessor` by checking
`self.location` to not be `None` before accessing it:

- MountingAccessor's self.location changes between `str` and `None`
  depending on the state of the MountingAccessor object (initialized,
  started, stopped/finished). The assert lets checkers know it is str
  when it is used in these locations.

Signed-off-by: Bernhard Kaindl <bernhard.kaindl@cloud.com>

Author: bernhardkaindl

xcp/accessor.py

@@ -155,11 +155,13 @@ def finish(self):
             return
         self.start_count -= 1
         if self.start_count == 0:
+            assert self.location
             mount.umount(self.location)
             os.rmdir(self.location)
             self.location = None
 
     def writeFile(self, in_fh, out_name):
+        assert self.location
         logger.info("Copying to %s" % os.path.join(self.location, out_name))
         out_fh = open(os.path.join(self.location, out_name), "wb")
         return self._writeFile(in_fh, out_fh)