x86: do not run vmx_pf_exception_test twice

vmx_pf_exception_test runs for quite some time, therefore it has its own
separate stanza in x86/unittests.cfg.  However, the main "vmx" test
does not skip it, thus causing the test to run twice.  Remove it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>