kvm: x86: Add request bit to reload APIC access page address

Currently, the APIC access page is pinned by KVM for the entire life
of the guest.  We want to make it migratable in order to make memory
hot-unplug available for machines that run KVM.

This patch prepares to handle this in generic code, through a new
request bit (that will be set by the MMU notifier) and a new hook
that is called whenever the request bit is processed.

Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>