KVM: nVMX: vmread/vmwrite: Use shadow vmcs12 if running L2

This is done as a preparation to VMCS shadowing emulation.

Signed-off-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>