Bridging: Layer 2 bridge domains and virtual switching

Add support for Layer 2 bridge domains and virtual switches. Like a VLAN, a bridge domain spans one or more ports on multiple devices and can be created within a user-created virtual switch or the default virtual switch. Additionally, a virtual switch can run one of the spanning-tree protocols (see the subsequent item titled "Spanning-tree protocols support for Layer 2 bridging and VPLS") and spanning-tree bridge protocol data units (BPDUs) can be tunneled.