KVM: MMU: introduce is_last_spte helper

Hiding some of the last largepage / level interaction (which is useful
for gbpages and for zero based levels).

Also merge the PT_PAGE_TABLE_LEVEL clearing loop in unlink_children.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>