vchan grant issue

```irc
<user> hmm, i have the agent get stuck after a while. 
<user> 2018-01-17 15:35:04 -00:00: WRN [qubes.rexec] Error setting up vchan (domain 167, port 516): (Failure "stub_gnttab_mapv_batched: failed to map grant ref") 
<user> and while shutting down, it reports several like this: 
<user> port 7 still bound!
```