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
/lib/systemd/system/qemu-guest-agent.service, which uses the invalid
path, /usr/bin/qemu-ga.
Two workarounds for now:-
* Edit /lib/systemd/system/qemu-guest-agent.service to use the qemu.ga
correct path.
* Make a symlink, i.e. ln -s /usr/sbin/qemu-ga /usr/bin/qemu-ga
