qemu: agent: give better error messages whe agent monitor is down

We can receive NULL as sync reply in two situations. First
is garbage sync reply and this situation is handled by
resending sync message. Second is different cases
of rebooting guest, destroing domain etc and we can
give more meaningful error message. Actually we have
this error message in qemuAgentCommand already which checks
for the same sitatuion. AFAIK case with mon->running
is just to be safe on adding some future(?) cases of
returning NULL reply.

src/qemu/qemu_agent.c

@@ -992,8 +992,12 @@ qemuAgentGuestSync(qemuAgentPtr mon)
             memset(&sync_msg, 0, sizeof(sync_msg));
             goto retry;
         } else {
-            virReportError(VIR_ERR_AGENT_UNSYNCED, "%s",
-                           _("Missing monitor reply object"));
+            if (mon->running)
+                virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                               _("Missing monitor reply object"));
+            else
+                virReportError(VIR_ERR_AGENT_UNRESPONSIVE, "%s",
+                               _("Guest agent disappeared while executing command"));
             goto cleanup;
         }
     }