WIP: Machine.WatchInterfaces NotifyWatcher

state:WatchInterfaces

This is a NotifyWatcher that monitors the machine's network interfaces, and reports the Ids of network interfaces that are modified.

This should catch Added, Removed, Enabled, Disabled
// machineInterfacesWatcher notifies about changes to all network
// interfaces of a machine. Changes include adding, removing,
// enabling or disabling interfaces.

Then I'd drop the second part of the comment.