vmnetx.ui: Avoid race if startup is cancelled very late

which can cause the cancellation request to be ignored.  When the UI
receives the vm-started signal from the controller, notice this and
properly shut down the VM.
Revert "LocalController: Avoid race if startup is cancelled very early"

LocalController._launch_lock was preventing the stop thread from actually
stopping the VM until startup was finished, thereby making it impossible
to actually cancel startup.  The UI now has its own code to recover from
cancellation races, so rely on that to handle the early-cancellation case.

This reverts commit 6c5a3a47696b84041a714187578522fa4f8a14f5.