Introduced by QEMU commit 98fc1ada4cf70af0f1df1a2d7183cf786fc7da05
    virtio: add vhost-user-fs base device

Released in QEMU v4.2.0.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NPeter Krempa <pkrempa@redhat.com>
Acked-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Tested-by: NAndrea Bolognani <abologna@redhat.com>