When I reinstalled Virtualizor I tried again to use the optional lvg parameter. It apparently wants the the vg specified as /dev/vg. The correct syntax for my installation was
./install.sh email=your@email.com kernel=kvm lvg=/dev/vg
After installing net-tools on the server, which provides the ifconfig and route commands required by 'service virtnetwork start' (and re-entering the name of my network interface in the Virtualizor config, as above) the 'service virtnetwork start' successfully starts the viifbr0 bridge required by a KVM vps.