KVM: x86: add track_flush_slot page track notifier

When a memory slot is being moved or removed users of page track
can be notified. So users can drop write-protection for the pages
in that memory slot.

This notifier type is needed by KVMGT to sync up its shadow page
table when memory slot is being moved or removed.

Register the notifier type track_flush_slot to receive memslot move
and remove event.

Reviewed-by: Xiao Guangrong <guangrong.xiao@intel.com>
Signed-off-by: Chen Xiaoguang <xiaoguang.chen@intel.com>
[Squashed commits to avoid bisection breakage and reworded the subject.]
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>