vmm: Add option to control backing files

Backing files (e.g. for QCOW2) interact badly with landlock since they
are not obvious from the initial VM configuration. Only enable their use
with an explicit option.

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

vmm/src/vm_config.rs

@@ -284,6 +284,8 @@ pub struct DiskConfig {
     pub serial: Option<String>,
     #[serde(default)]
     pub queue_affinity: Option<Vec<VirtQueueAffinity>>,
+    #[serde(default)]
+    pub backing_files: bool,
 }
 
 impl ApplyLandlock for DiskConfig {