KVM: x86: Make CR4.VMXE reserved for the guest

CR4.VMXE is reserved unless the VMX CPUID bit is set.  On Intel,
it is also tested by vmx_set_cr4, but AMD relies on kvm_valid_cr4,
so fix it.

Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>