Add LXD VM `STATE` field for LXD Agent status


In order to manage the guest operating system of an LXD virtual machine, the LXD Agent software needs to be running on the guest. For example, you cannot use the lxc exec command unless the LXD Agent is up and running.

However, the STATE field returned by the lxc list command doesn’t provide any indication that the LXD Agent is running / healthy. The STATE field only indicates that the VM itself is running or not.

I think it would be useful to include an additional state, such as HEALTHY to indicate that the VM is not only running, but also that the LXD Agent is connected to the LXD daemon / engine.

What do you think?