Fix two race conditions in reboot instance

If the instance crashes between backend.InstanceReboot checks the list
of running instances and the execution of hv_xen.RebootInstance,
ini_info will be None. And if the instance doesn't reboot fast enough,
new_info will be None. Both cases lead to “TypeError: unsubscriptable
object”. Too bad pylint doesn't detect such cases.

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