vmm: api: Make FsConfig defaults match between CLI and HTTP API

In order to let the CLI and the HTTP API behave the same regarding the
FsConfig structure, this patch defines some default values for
num_queues, queue_size and the cache_size.

num_queues is set to 1, queue_size is set to 1024, and cache_size is set
to Some(8G) which means that DAX is enabled by default with a shared
region of 8GiB.

Fixes #508

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

vmm/src/api/openapi/cloud-hypervisor.yaml

@@ -312,8 +312,6 @@ components:
       required:
       - tag
       - sock
-      - num_queues
-      - queue_size
       type: object
       properties:
         tag:
@@ -322,11 +320,14 @@ components:
           type: string
         num_queues:
           type: integer
+          default: 1
         queue_size:
           type: integer
+          default: 1024
         cache_size:
           type: integer
           format: int64
+          default: 8589934592
 
     PmemConfig:
       required: