vhost-user: fix ioeventfd_enabled

Currently, the vhost-user-test assumes the eventfd is available.
However it's not true because the accel is qtest. So the
'vhost_set_vring_file' will not add fds to the msg and the server
side of vhost-user-test will be broken. The bug is in 'ioeventfd_enabled'.
We should make this function return true if not using kvm accel.
Signed-off-by: NLi Qiang <liq3ea@163.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost-user.c

@@ -207,7 +207,7 @@ struct vhost_user {
 
 static bool ioeventfd_enabled(void)
 {
-    return kvm_enabled() && kvm_eventfds_enabled();
+    return !kvm_enabled() || kvm_eventfds_enabled();
 }
 
 static int vhost_user_read(struct vhost_dev *dev, VhostUserMsg *msg)