kvm: mmu: Add guest_mode to kvm_mmu_page_role

L1 and L2 need to have disjoint mappings, so that L1's APIC access
page (under VMX) can be omitted from L2's mappings.

Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>