x86: fixup machine_ops reboot_{32|64}.c unification fallout

When reboot_32.c and reboot_64.c were unified (commit 4d022e35

...),
the machine_ops code was broken, leading to xen pvops kernels failing
to properly halt/poweroff/reboot etc. This fixes that up.

Signed-off-by: Jody Belka <knew-linux@pimb.org>
Cc: Miguel Boton <mboton@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>