statemachine: goToState() should require that the machine is running

goToState() is an internal function. Allowing both goToState() and
machine.setInitialState() to be used to set the initial state of the
machine complicates the logic of QStateMachine::start().

The existing tests for goToState() still pass.

Change-Id: Ie831b4c869848f7f4c3e6bd576cf298a9799eb22
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>