Try to reduce wrong errors in InstanceShutdown

In backend.InstanceShutdown(), there is a race condition between
checking that the instance exists and trying to shut it down which
translates sometime in error messages like:

Tue Oct 20 20:08:30 2009 - WARNING: Could not shutdown instance: Failed
to force stop instance instance9: Failed to stop instance instance9:
exited with exit code 1, Error: Domain 'instance9' does not exist.

To fix this, we ignore any hypervisor StopInstance() errors if the
instance doesn't exist anymore, since our purpose (to make the instance
go away) is already accomplished.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>