KVM: Avoid calling smp_call_function_single() with interrupts disabled

When taking a cpu down, we need to hardware_disable() it.
Unfortunately, the CPU_DYING notifier is called with interrupts
disabled, which means we can't use smp_call_function_single().

Fortunately, the CPU_DYING notifier is always called on the dying cpu,
so we don't need to use the function at all and can simply call
hardware_disable() directly.

Tested-by: Paolo Ornati <ornati@fastwebnet.it>
Signed-off-by: Avi Kivity <avi@qumranet.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>