network: move re-attach of bridge device out of network driver
    
    During initial NIC setup the hypervisor drivers are responsible for
    attaching the TAP device to the bridge device. Any fixup after libvirtd
    restarts should thus also be their responsibility.
    
    Reviewed-by: Laine Stump <laine@laine.org>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>