KVM: VMX: emulate accessed bit for EPT

Currently KVM pretends that pages with EPT mappings never got
accessed.  This has some side effects in the VM, like swapping
out actively used guest pages and needlessly breaking up actively
used hugepages.

We can avoid those very costly side effects by emulating the
accessed bit for EPT PTEs, which should only be slightly costly
because pages pass through page_referenced infrequently.

TLB flushing is taken care of by kvm_mmu_notifier_clear_flush_young().

This seems to help prevent KVM guests from being swapped out when
they should not on my system.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>