x86: svm: Skip NPT-only part of guest CR3 tests when NPT is disabled

Skip the sub-tests for guest CR3 that rely on NPT, unsurprisingly they
fail when running with NPT disabled.  Alternatively, the test could be
modified to poke into the legacy page tables, but obviously no one
actually cares that much about shadow paging.

Fixes: 6d0ecbf6 ("nSVM: Test non-MBZ reserved bits in CR3 in long mode and legacy PAE mode")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210422025448.3475200-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>