This is the package message:
# pkg install qemu-guest-agent
Modify your `/etc/rc.conf` by adding these settings
qemu_guest_agent_flags="-d -v -l /var/log/qemu-ga.log"
# service qemu-guest-agent start
For detailed help use `qemu-ga -h`
…to which my first reaction is: if that's the correct value to use, why saying it in the message instead of actually using it as a default in the rc.d?
Turns out the comment in the rc.d explains better: the default is fine with only "-d", you can add "-v -l …" if you want more logs; which makes sense to me, but in that case I'd suggest re-wording the pkg-message (or removing it altogether, how to enable and start a service is pretty common information IMHO, nothing specific to this package).