vmm/virtualbox: Improve handling of save file at unregistration
vmm/virtualbox: Improve handling of save file at unregistration

- Rename the save file out of the way rather than copying it, to save I/O

- The VM's stateFilePath can be equal to saved_path at unregistration
time if the VM was never successfully started.  To prevent the save
file from being lost in this case, rename the save file to a temporary
path before discardSavedState() and to the permanent path afterward.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr