fix CPUID vendor override

the meaning of vendor_override is actually the opposite of how it
is currently used :-(
Fix it to allow KVM to export the non-native CPUID vendor if
explicitly requested by the user.

The intended behavior is:
With TCG:
  - always inject the configured vendor (either hard-coded, in config
    files or via ",vendor=" commandline)
With KVM:
  - by default inject the host's vendor
  - if the user specifies ",vendor=" on the commandline, use this
    instead of the host's vendor
  - all pre-configured vendors (hard-coded, config file) are ignored
Signed-off-by: NAndre Przywara <andre.przywara@amd.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

target-i386/cpuid.c

@@ -982,7 +982,7 @@ static void get_cpuid_vendor(CPUX86State *env, uint32_t *ebx,
      * this if you want to use KVM's sysenter/syscall emulation
      * in compatibility mode and when doing cross vendor migration
      */
-    if (kvm_enabled() && env->cpuid_vendor_override) {
+    if (kvm_enabled() && ! env->cpuid_vendor_override) {
         host_cpuid(0, 0, NULL, ebx, ecx, edx);
     }
 }