After installing qemu-guest-agent with yum, which was easy enough with sudo yum install qemu-guest-agent I did not see IP information in the proxmox host, so I knew it was not yet working. I tried several things, none of them worked. First I tried looking at the status. Got an error indicating the service was not installed:
# sudo service qemu-ga status
Redirecting to /bin/systemctl status qemu-ga.service
Unit qemu-ga.service could not be found.
So then I tried to reinstall, but got a notice that it was already installed:
# sudo yum install qemu-guest-agent
Package 10:qemu-guest-agent-2.12.0-3.e17.x86_64 already installed and latest version
Nothing to do
So I tried to start the service, got an error that it didn't exist:
# sudo chkconfig qemu-ga on
Error reading information on service qemu-ga: No such file or directory
Same with service qemu-ga start and so forth.
Startpage search turned up this hit in the proxmox forums.
And Proxmox also recommends reboot for "some distros."
So I rebooted. Seemed like a lot for a little, but, rebooting the guest server solved the problem.
Note, the two error messages for status and chkconfig still appear after reboot, even though the guest agent is working properly.