vmm: config: add memory_restore_mode to RestoreConfig

Add a MemoryRestoreMode enum (Copy | OnDemand) to RestoreConfig so
the restore path can be selected at restore time. Copy preserves the
existing eager read-copy behavior. OnDemand enables userfaultfd-based
demand paging and fails restore if the kernel does not support it.

Validate that prefault=on is not combined with OnDemand mode.

Update the OpenAPI spec with the new enum field.

Signed-off-by: Shayon Mukherjee <shayonj@gmail.com>

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

@@ -1342,6 +1342,11 @@ components:
         destination_url:
           type: string
 
+    MemoryRestoreMode:
+      type: string
+      enum: [Copy, OnDemand]
+      default: Copy
+
     RestoreConfig:
       required:
         - source_url
@@ -1351,6 +1356,8 @@ components:
           type: string
         prefault:
           type: boolean
+        memory_restore_mode:
+          $ref: "#/components/schemas/MemoryRestoreMode"
 
     ReceiveMigrationData:
       required: