KVM: MMU: do not reuse the obsolete page

The obsolete page will be zapped soon, do not reuse it to
reduce future page fault

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>