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: Alex Bennée <alex.bennee@linaro.org>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Roman Kagan <rkagan@virtuozzo.com>
Message-Id: <20190624123835.28869-1-alex.bennee@linaro.org>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>