Currently, the VM controller instantiates the network admitter as part of its startVMI flow [1].

Decouple it from the VM controller and add a unit test to make sure its being called.

Check if there is a need to change the field to `spec.template.spec`.

 [1]https://github.com/kubevirt/kubevirt/blob/f817fe67692c71801d72633d7f5079395d727e60/pkg/virt-controller/watch/vm/vm.go#L1405