Use the NX bit from the MMU's role instead of the MMU itself so that the
redundant, dedicated "nx" flag can be dropped.

No functional change intended.
Signed-off-by: NSean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-36-seanjc@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>