nEPT: make guest's A/D bits depends on guest's paging mode

This patch makes guest A/D bits definition to be dependable on paging
mode, so when EPT support will be added it will be able to define them
differently.

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