virtio-devices: balloon: Enable use with confidential VMs

Following the pattern used by the existing virtio devices make the
balloon device work with confidential VMs (e.g. SEV-SNP). This requires
advertising the VIRTIO_F_ACCESS_PLATFORM feature. Do not expose this to
the user as a controllable option and instead only enable in on the
"force" case.

Signed-off-by: Rob Bradford <rbradford@meta.com>

vmm/src/device_manager.rs

@@ -3704,6 +3704,7 @@ impl DeviceManager {
                     balloon_config.size,
                     balloon_config.deflate_on_oom,
                     balloon_config.free_page_reporting,
+                    self.force_access_platform,
                     self.seccomp_action.clone(),
                     self.exit_evt
                         .try_clone()