nSVM: Explicitly save/update/restore EFER.NX for NPT NX test

Explicitly set EFER.NX in the NPT NX test instead of assuming all tests
will run with EFER.NX=1, and use the test's scratch field to save/restore
EFER.  There is no need to force EFER.NX=1 for all tests, and a future
test will verify that a #NPT occurs when EFER.NX=0 and PTE.NX=1, i.e.
wants the exact opposite.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622210047.3691840-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>