Add CleanupInstance hypervisor call

Currently some hypervisors (namely kvm) need to do some cleanup after
making sure an instance is stopped. With the moving of the retry cycle
in backend those cleanups were never done. In order to solve this we add
a new optional hypervisor function, CleanupInstance, which gets called
at the end of the shutdown procedure, and which interested hypervisors
can implement to be sure not to miss cleanup operations.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>