vmx: remove unnecessary #ifdef __x86_64__

The VMX tests are 64-bit only, so checking the architecture is
unnecessary.  Also, if the tests supported 32-bits environments
the #ifdef would probably go in test_canonical.

Reported-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>