Move flag in libvirt's vif to the libvirt group

This change continues moving libvirt specific flags into a libvirt
specific configuration group.

Progresses blueprint libvirt-opts-group.

DocImpact: the following flag has been moved into the libvirt
group:
    use_virtio_for_bridges (formerly libvirt_use_virtio_for_bridges)

Change-Id: I755139f0a470a091e36760836c9200f15aa72c37
Merge "Move flag in libvirt's vif to the libvirt group"