Support FlatDHCP network for VMware Hyper.

Auto create bridge when using FlatDHCP network, Add unittests for vif.

Fix bug #1171783
Fix bug #1177635

Change-Id: Ibb75c8e3174283f0f5e94bacfd61ee93e8bd6227
(cherry picked from commit bc66b8aa968a0c0b74e2f21ed0fc856f8933c6d3)
Merge "Support FlatDHCP network for VMware Hyper." into stable/grizzly