KVM: MMU: introduce a common function to get no-dirty-logged slot

Cleanup the code of pte_prefetch_gfn_to_memslot and mapping_level_dirty_bitmap

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