Its a bug with Centos 7. Vmin is not able to start ClamAV and you must do it manually with “systemctl start clamd@scan”. After you start it manually you must do the same in Wmin - bootup module if not ClamAV will not start on next server reboot.
Nuts, I wonder if that issue is related to the missing packages issue described here. Could you take a peek at this bug report, and see if the steps here allow you to start and stop ClamAV from within Virtualmin: