x86: nVMX: fix EPT A/D disabled MMIO access test

When testing MMIO interface to APIC we need to make sure that APIC is in
xAPIC mode (and not x2APIC). KVM started enforcing this since commit
d1766202779e ("x86/kvm/lapic: always disable MMIO interface in x2APIC
mode").

While the 'clean' fix would be to separate L2's APIC from L1's and put
it to xAPIC mode or create a different MMIO mapping just for the test
we lack the required infrastructure in vmx library. As a quick band-aid
put L1's APIC to xAPIC mode for both EPT A/D enabled and disabled tests,
it seems we don't require x2APIC for anything.

Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>