python/qemu/qmp: assert sockfile is not None

In truth, if you don't do this, you'll just get a TypeError
exception. Now, you'll get an AssertionError.

Is this tangibly better? No.
Does mypy complain less? Yes.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200514055403.18902-21-jsnow@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

python/qemu/qmp.py

@@ -94,6 +94,7 @@ def __negotiate_capabilities(self):
         raise QMPCapabilitiesError
 
     def __json_read(self, only_event=False):
+        assert self.__sockfile is not None
         while True:
             data = self.__sockfile.readline()
             if not data: