Comment 7 for bug 1878973

I got this reproduced when retrying again for another case-
Actually not too hart to hit :-/

Taking a Focal system with the version of qemu-guest-agent you reported and then making sure it is installed, up and running:

● qemu-guest-agent.service - QEMU Guest Agent
     Loaded: loaded (/lib/systemd/system/qemu-guest-agent.service; static; vendor preset: enabled)
     Active: active (running) since Tue 2020-06-02 05:25:04 UTC; 3min 8s ago
   Main PID: 624 (qemu-ga)
      Tasks: 1 (limit: 533)
     Memory: 900.0K
     CGroup: /system.slice/qemu-guest-agent.service
             └─624 /usr/sbin/qemu-ga

Pass a shutdown command through virsh:
  $ virsh shutdown focal --mode agent

The guest almost immediately went down, without "functional" issues.
But when checking the logs later I found:
Jun 02 05:29:54 focal qemu-ga[624]: ERROR:/build/qemu-74sXTC/qemu-4.2/qga/main.c:532:send_response: assertion failed: (rsp && s->channel)
Jun 02 05:29:54 focal qemu-ga[624]: Bail out! ERROR:/build/qemu-74sXTC/qemu-4.2/qga/main.c:532:send_response: assertion failed: (rsp && s->channel)