Implement flag.Value interface in NetworkFlag

The '-net' option is now vm.create specific.
Add HostNetworkSystem wrapper
Add HostConfigManager wrapper
Add HostSystem ConfigManager getter
Add SearchFlag HostNetworkSystem helper
Add host.vswitch.add cli command
Include host/vswitch commands
Add host.vswitch.info command
Add host.vswitch.remove command
Add host.portgroup.add command
Add host.portgroup.remove command
Import host.portgroup commands
Add vm.network.add command
Add vm.network.remove command
Import vm.network commands
Merge pull request #97 from dougm/host-network

Host network related wrappers and govc commands