core: properly redirect qrexec output to /dev/null when necessary

/dev/null was opened in read-only mode ("rw" is invalid value), so
qrexec couldn't write there.

core-modules/000QubesVm.py

@@ -1527,7 +1527,7 @@ class QubesVm(object):
 
         call_kwargs = {}
         if ignore_stderr or not passio:
-            null = open("/dev/null", "rw")
+            null = open("/dev/null", "w+")
             call_kwargs['stderr'] = null
         if not passio:
             call_kwargs['stdin'] = null