[Bridge + 8021q] vif_add/vif_del Switch API functions

* create VLAN interface specific data on vif_add and remove it of
vif_del;
* add a bridge both on vlan_add and vif_add if it hasn't been
added before;
* the bridge is removed when the VLAN and it's virtual interface
are deleted.

Signed-off-by: Claudiu Ghioc <claudiu.ghioc@gmail.com>