Revert "vmm: Deprecate MemoryZoneConfig::file"

This reverts commit 9fb0274479.

A user was identified of this functionality.

See: #4837
Signed-off-by: Rob Bradford <robert.bradford@intel.com>

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

@@ -668,6 +668,8 @@ components:
           type: integer
           format: int64
           default: 512 MB
+        file:
+          type: string
         mergeable:
           type: boolean
           default: false