Bug#918378: fixed in qemu 1:3.1+dfsg-3
Just updated my test machines to the newly released qemu/1:3.1+dfsg-4 and can confirm it's fixed. Thanks!
Bug#918378: (no subject)
I just stumbled upon the same issue when building some new Debain 10 virtual machines (with Qemu 1:3.1+dfsg-2). In Debian 9, the Qemu Guest Agent was started by /etc/init.d/qemu-guest-agent, which used the valid path, /usr/sbin/qemu-ga. In Debian 10, it is started by the systemd script