VBOX: Make the service install scenario work again.

VBOX: When checking for the existence of a VM registered to a given slot directory, also check for the ISO Contextualization image and cache disk image.  If any type of media is registered to a given slot directory, close it regardless if it is attached to a VM or not.

VBOX: last_checkpoint_cpu_time != last_checkpoint_elapsed_time.  When to checkpoint is based on elapsed time, what we reported to boinc is the cpu time.  Keep track of both values.

VBOX: When deregistering a VM, organize the medium list by most recent child first and then release as needed.

VBOX: Introduce the notion of Virtualbox COM namespaces. We currently support VirtualBox 4.2 and VirtualBox 4.3.

VBOX: Do not attempt to restore the current snapshot if the VM hasn't been running long enough to generate one.

VBOX: If something fails during create_vm(), we must release the write lock before deregister_vm() is called. Handle machine and session cleanup at the end if the routine.

When the file is detected, the wrapper will call boinc_temporary_exit() and restart the VM at a later date.

VBOX: Move common code to VBOX_BASE class, VBOX_VM derives from it by extending virtual functions calling Vboxmanage or Native API functions.  Use ATL on Windows to wrap COM Objects and data types.

VBOX: Move common code to VBOX_BASE class, VBOX_VM derives from it by extending virtual functions calling Vboxmanage or Native API functions.  Use ATL on Windows to wrap COM Objects and data types.