Fix unwanted closing of libvirt client connection

e5a1bee07 introduced a regression in Boxes: when Boxes is left idle
(it's still doing some libvirt calls in the background), the
libvirt connection gets closed after a few minutes. What happens is
that this code in virNetClientIOHandleOutput gets triggered:

if (!thecall)
    return -1; /* Shouldn't happen, but you never know... */

and after the changes in e5a1bee07, this causes the libvirt connection
to be closed.

Upon further investigation, what happens is that
virNetClientIOHandleOutput is called from gvir_event_handle_dispatch
in libvirt-glib, which is triggered because the client fd became
writable. However, between the times gvir_event_handle_dispatch
is called, and the time the client lock is grabbed and
virNetClientIOHandleOutput is called, another thread runs and
completes the current call. 'thecall' is then NULL when the first
thread gets to run virNetClientIOHandleOutput.

After describing this situation on IRC, danpb suggested this:

11:37 < danpb> In that case I think the correct thing would be to change
               'return -1' above to 'return 0' since that's not actually an
               error - its a rare, but expected event

which is what this patch is doing. I've tested it against master
libvirt, and I didn't get disconnected in ~10 minutes while this
happens in less than 5 minutes without this patch.

src/rpc/virnetclient.c

@@ -1205,7 +1205,10 @@ virNetClientIOHandleOutput(virNetClientPtr client)
         thecall = thecall->next;
 
     if (!thecall)
-        return -1; /* Shouldn't happen, but you never know... */
+        return 0; /* This can happen if another thread raced with us and
+                   * completed the call between the time this thread woke
+                   * up from poll()ing and the time we locked the client
+                   */
 
     while (thecall) {
         ssize_t ret = virNetClientIOWriteMessage(client, thecall);