config: Add option dax and cache_size to virtio-fs

In order to support the more performant version of virtio-fs, that is
the one relying on a shared memory region between host and guest, we
introduce two new parameters to the --fs device.

The "dax" parameter allows the user to choose if he wants to use the
shared memory region with virtio-fs. By default, this parameter is "on".

The "cache_size" parameter allows the user to specify the amount of
memory that should be shared between host and guest. By default, the
value of this parameter is 8Gib as advised by virtio-fs maintainers.

Note that dax=off and cache_size are incompatible.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

src/main.rs

@@ -119,7 +119,8 @@ fn main() {
                 .help(
                     "virtio-fs parameters \"tag=<tag_name>,\
                      sock=<socket_path>,num_queues=<number_of_queues>,\
-                     queue_size=<size_of_each_queue>\"",
+                     queue_size=<size_of_each_queue>,dax=on|off,\
+                     cache_size=<DAX cache size: default 8Gib>\"",
                 )
                 .takes_value(true)
                 .min_values(1),