Give shim implementor ability to specify action after changing to normal mode

[sevenautumns]

As of right now, after cold/warm-start apex-rs transitions to normal mode and then enters a busy loop.
Some hypervisors may or may not stop the main process but require some other mechanism.
For this we should offer a default implementation of the "busy-loop" loop{} but give the option to overwrite this with the ApexPartitionP4 trait.