If the openvswitch-service-type is part of the operating-system services
then virt-manager should be able to add virtual network devices on a
per-VM basis.
Here's the services section from my machine hosting VMs:
(services (append (list (static-networking-service
"enp3s0f0" "192.168.1.5"
#:netmask "255.255.255.0"
#:gateway "192.168.1.254")
(lsh-service #:interfaces '("192.168.1.5"))
(service openvswitch-service-type
(openvswitch-configuration))
(service libvirt-service-type
(libvirt-configuration
(unix-sock-group "wheel")
(listen-addr "192.168.1.5")
(listen-tls? #f)
(listen-tcp? #t)))
(service virtlog-service-type))
%custom-services))
