[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>
[x86] Make arch_defterm_getc() sleepable.

The arch_defterm_getc() is always called in Orphan (or Thread) context
so as a small optimization we sleep waiting for input characters in
defterm fifo.

Also, its quite inefficient to constantly poll defterm fifo.

Signed-off-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Himanshu Chauhan <hschauhan@nulltrace.org>
Merge x86-next from https://github.com/hschauhan/xvisor-x86_64