Mini-OS: cleanup hypervisor.c

Mini-OS: cleanup hypervisor.c

Do the following cleanups in hypervisor.c:

- Let hypervisor.c conform to the coding style.
- Drop the bogus "inline" attributes of exported functions.
- Replace a always zero cpu variable with smp_processor_id() as
  elsewhere in the code.
- Replace "if () BUG()" with BUG_ON() in case the condition has no
  side effects.
- Drop the unused cpu parameter from the active_evtchns() macro.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>