vmm: Introduce new --balloon parameter

This introduces a new way of defining the virtio-balloon device. Instead
of going through the --memory parameter, the idea is to consider balloon
as a standalone virtio device.

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

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

@@ -410,6 +410,8 @@ components:
             $ref: '#/components/schemas/NetConfig'
         rng:
           $ref: '#/components/schemas/RngConfig'
+        balloon:
+          $ref: '#/components/schemas/BalloonConfig'
         fs:
           type: array
           items:
@@ -647,6 +649,15 @@ components:
           type: boolean
           default: false
 
+    BalloonConfig:
+      required:
+      - size
+      type: object
+      properties:
+        size:
+          type: integer
+          format: int64
+
     FsConfig:
       required:
       - tag