target/i386: fix feature check in hyperv-stub.c

Commit 2d384d7c broken the build when built with:

  configure --without-default-devices --disable-user

The reason was the conversion of cpu->hyperv_synic to
cpu->hyperv_synic_kvm_only although the rest of the patch introduces a
feature checking mechanism. So I've fixed the KVM_EXIT_HYPERV_SYNIC in
hyperv-stub to do the same feature check as in the real hyperv.c
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NRoman Kagan <rkagan@virtuozzo.com>
Reviewed-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>

target/i386/hyperv-stub.c

@@ -15,7 +15,7 @@ int kvm_hv_handle_exit(X86CPU *cpu, struct kvm_hyperv_exit *exit)
 {
     switch (exit->type) {
     case KVM_EXIT_HYPERV_SYNIC:
-        if (!cpu->hyperv_synic) {
+        if (!hyperv_feat_enabled(cpu, HYPERV_FEAT_SYNIC)) {
             return -1;
         }