KVM: MMU: store generation-number into mmio spte

Store the generation-number into bit3 ~ bit11 and bit52 ~ bit61, totally
19 bits can be used, it should be enough for nearly all most common cases

In this patch, the generation-number is always 0, it will be changed in
the later patch

[Gleb: masking generation bits from spte in get_mmio_spte_gfn() and
       get_mmio_spte_access()]

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Reviewed-by: Gleb Natapov <gleb@redhat.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>