[x86] Register reset & shutdown callbacks

The arch_board_reset() and arch_board_shutdown() are
depricated.

Now we have to use vmm_register_system_reset() and
vmm_register_system_shutdown() to provide reset & shutdown
callbacks.

This patch fixes x86_64_generic board as per above.

Signed-off-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Himanshu Chauhan <hschauhan@nulltrace.org>
Acked-by: Himanshu Chauhan <hschauhan@nulltrace.org>