Hypervisors: Add retry= to StopInstance

Currently some hypervisors need the stop operations to be retried more
than once, while other ones only do it in one pass. With this change
we'll handle retries outside the hypervisor code, but telling whether
this is the first try or not.

Since this option is not used for now, all hypervisors just return if
called with retry set to on, maintaining the old behavior. Since the
fake hypervisor has an idempotent StopInstance call, we avoid returning
in that case.

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