Comment 9 for bug 1820291

Chances are that affected people have fixed it up in
  /etc/default/qemu-guest-agent
just as you have or in latter releases modified (the defaults file was dropped)
  /lib/systemd/system/qemu-guest-agent.service

Now exactly those (few) people that would have fixed the path up that way would be broken by an update moving the file.

While I admit the path is wrong, changing it might cause more pain than it would help.
So instead we should leave the path as is, but instead change the default config in a way that works.

That means adding the path that it has on disk to the default-config/systemd file.

I'm only 90% sure on this, so I beg your pardon if I make up my mind differently later on :-)