Using kvm_mmu_prepare_zap_page() and kvm_mmu_commit_zap_page() to
split kvm_mmu_zap_page() function, then we can:

- traverse hlist safely
- easily to gather remote tlb flush which occurs during page zapped

Those feature can be used in the later patches
Signed-off-by: NXiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>