KVM: nVMX: Don't speculatively write virtual-APIC page address

The VIRTUAL_APIC_PAGE_ADDR in vmcs02 is guaranteed to be updated before
it is consumed by hardware, either in nested_vmx_enter_non_root_mode()
or via the KVM_REQ_GET_VMCS12_PAGES callback.  Avoid an extra VMWRITE
and only stuff a bad value into vmcs02 when mapping vmcs12's address
fails.  This also eliminates the need for extra comments to connect the
dots between prepare_vmcs02_early() and nested_get_vmcs12_pages().

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>