After a zimbra restart i had the same problem so i did a little hack and now everything is working fine even after zimbra restarts. I created an empty file
/opt/zimbra/log/clamd.pid.fake as zimbra user. Then as root user i inserted
in /opt/zimbra/bin/zmantivirusctl right after the line
cp /opt/zimbra/log/clamd.pid.fake /opt/zimbra/log/clamd.pid
and now everything works fine since everytime clamav starts it creates the clamd.pid file from clamd.pid.fake.
/opt/zimbra/bin/$i start norewrite
Be sure to backup the /opt/zimbra/bin/zmantivirusctl file before making any change just in case something goes wrong