Add "reboot_behavior" hypervisor flag

During instance installations, you do not want the instance to reboot
and start again with the same parameters, as that will most likely
re-start the install process. Therefore, when the instance requests a
reboot it should instead shutdown. This flag allows this to be
controlled.

Signed-off-by: Stephen Shirley <diamond@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>