vmm: openapi: Sync DiskConfig OpenAPI spec

Add backing_files field to the REST API.

Note: This backport does not include the 'sparse' field as the
corresponding feature support is not backported.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
(cherry picked from commit e36096db3e)

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

@@ -941,6 +941,9 @@ components:
           type: array
           items:
             $ref: "#/components/schemas/VirtQueueAffinity"
+        backing_files:
+          type: boolean
+          default: false
 
     NetConfig:
       type: object