KVM: update mmu documetation for role.nxe

There's no member "cr4_nxe" in struct kvm_mmu_page_role, it names "nxe" now.
Update mmu document.

Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>