vmm: api: Modify FsConfig to be OpenAPI friendly

When consumer of the HTTP API try to interact with cloud-hypervisor,
they have to provide the equivalent of the config structure related to
each component they need. Problem is, the Rust enum type "Option" cannot
be obtained from the OpenAPI YAML definition.

This patch intends to fix this inconsistency between what is possible
through the CLI and what's possible through the HTTP API by using simple
types bool and int64 instead of Option<u64>.

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

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

@@ -321,9 +321,13 @@ components:
           type: integer
         queue_size:
           type: integer
+        dax:
+          type: boolean
+          default: true
         cache_size:
           type: integer
           format: int64
+          default: 8589934592
 
     PmemConfig:
       required: