CA-139501 VM hard_shutdown & hard_reboot must wait for nothing else.

CA-139501 VM hard_shutdown & hard_reboot must wait for nothing else.

We were already allowing hard_reboot concurrent with a clean_reboot
already in progress, and the same for hard/clean shutdown, but
clean_(shutdown|reboot) would block hard_(reboot|shutdown).

Now the is_allowed_concurrently function allows hard_shutdown
concurrent with anything except itself, and allows hard_reboot
concurrent with anything but itself or hard_shutdown.