Similar to the spice-vdagent (bug 876237), please enable the qemu-ga service by default. The package installs udev rules that should cause it to only be enabled when actually inside a VM with a proper virtio-serial setup.
What's the precise systemd service name we shall add?
$ cat /usr/lib/systemd/system/qemu-guest-agent.service
Description=QEMU Guest Agent
Commited this to the pkg git, didn't build this in koji though, but will be part of the next round of uploads.
Built in Rawhide (systemd-197-2.fc19), but not yet in F18.
I believe this is already fixed in systemd-197-1.fc18.2, I just forgot to link to this bug from the Bodhi update.
*** Bug 1165176 has been marked as a duplicate of this bug. ***