Get the MMU's effective EFER.NX from its role instead of using the
one-off, dedicated flag.  This will allow dropping said flag in a
future commit.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-38-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>