[Virtio-fs] False error message when DAX cache is disabled

[misono.tomohiro at fujitsu.com]

Hi,

I'm testing virtiofs on ARM enviroment.
Since current ARM does not support DAX, I use cache-size=0 for vhost-user-fs-pci parameter.

This results in the following error message during unmount:
```
vhost_user_fs_slave_unmap: unmap when DAX cache not present
lo_destroy: unmap during destroy failed
```

This is because lo_destroy() always calls fuse_virtio_unmap().
So, I want to fix this by not calling fuse_virtio_unmap() when cache is not used,
but is there any good way for virtiofsd to know vhost-user's cahce-size?

Thanks.
Misono