KVM: Drop user return notifier when disabling virtualization on a cpu

This way, we don't leave a dangling notifier on cpu hotunplug or module
unload.  In particular, module unload leaves the notifier pointing into
freed memory.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>