KVMHypervisor: use the StopInstance retry feature

Since we know StopInstance is going to be called more than once (at
least twice, once with force and once without, but normally quite a lot
more) we don't need our own sleep/loop, and we can just send one monitor
command per call.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>