vm/windows: Exit if VM has dropped out.

This ensures that the builder isn't waiting forever if the Windows VM
drops dead while we're waiting for the controller VM to signal that a
particular command has been executed on the Windows VM. It won't ever
happen in such cases so it doesn't make sense to wait for the timeout.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
vm/windows: Wait for migration to finish.

This ensures that the intermediate machine is shut down only after the
migration has finished writing the memory dump to disk, to ensure we
don't end up with empty state files depending on how fast the migration
finished before we actually shut down the VM.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>