The return value of vif_driver.plug() was used only when nova wrote the
libvirt.xml file on initial boot of the instance. Make it returns None,
and add get_config() to get the configurations if needed.
If LibvirtBridgeDriver is used when the nova-comptue inits, we call
vif_driver.plug() without get the VIF configurations. By doing this
plug doesn't raise if the instance has not yet got a fixed-ip. Fixes
bug 1076309.
Change-Id: I0fafb3d98e3139bce0ef2cee04cab428a0621696