KVM: MMU: Fix setting the accessed bit on non-speculative sptes

The accessed bit was accidentally turned on in a random flag word, rather
than, the spte itself, which was lucky, since it used the non-EPT compatible
PT_ACCESSED_MASK.

Fix by turning the bit on in the spte and changing it to use the portable
accessed mask.

Signed-off-by: Avi Kivity <avi@qumranet.com>