The QEMU guest agent protocol (QEMU GA) package, qemu-guest-agent, is fully supported in Red Hat Enterprise Linux 6.5 and newer. However, there are the following limitations with regards to isa-serial/virtio-serial transport:
The qemu-guest-agent cannot detect whether or not a client has connected to the channel.
There is no way for a client to detect whether or not qemu-guest-agent has disconnected or reconnected to the back-end.
If the virtio-serial device resets and qemu-guest-agent has not connected to the channel (generally caused by a reboot or hot plug), data from the client will be dropped.
If qemu-guest-agent has connected to the channel following a virtio-serial device reset, data from the client will be queued (and eventually throttled if available buffers are exhausted), regardless of whether or not qemu-guest-agent is still running or connected.