qemu_agent: Output newline at the end of the sync JSON message

Although this isn't apparently needed for the guest agent itself, the
test I will be adding later depends on the newline as a separator of
messages to process.

src/qemu/qemu_agent.c

@@ -918,7 +918,7 @@ qemuAgentGuestSync(qemuAgentPtr mon)
 
     if (virAsprintf(&sync_msg.txBuffer,
                     "{\"execute\":\"guest-sync\", "
-                    "\"arguments\":{\"id\":%llu}}", id) < 0)
+                    "\"arguments\":{\"id\":%llu}}\n", id) < 0)
         return -1;
 
     sync_msg.txLength = strlen(sync_msg.txBuffer);