KVM: MMU: fix apf prefault if nested guest is enabled

If apf is generated in L2 guest and is completed in L1 guest, it will
prefault this apf in L1 guest's mmu context.

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>