Merge pull request #156 from dougm/device-helpers

Add VirtualDeviceList FindByBackingInfo method
Remove vm.network.remove command

There can be multiple devices with the same network backing,
just use the device.remove command to remove ethernet card devices.
Use device name in vm.network.change command

There can be multiple devices with the same network backing,
so use the device name when changing networks.

Fixup so the network can be specified using either the '-net' flag or
Arg(1), as we do in the vm.network.add command.
Use DeviceAdd helper in vm.network.add command