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/config.rs

@@ -1093,7 +1093,7 @@ impl DiskConfig {
          ops_size=<io_ops>,ops_one_time_burst=<io_ops>,ops_refill_time=<ms>,\
          id=<device_id>,pci_segment=<segment_id>,rate_limit_group=<group_id>,\
          queue_affinity=<list_of_queue_indices_with_their_associated_cpuset>,\
-         serial=<serial_number>";
+         serial=<serial_number>,backing_files=on|off";
 
     pub fn parse(disk: &str) -> Result<Self> {
         let mut parser = OptionParser::new();
@@ -1118,7 +1118,8 @@ impl DiskConfig {
             .add("pci_segment")
             .add("serial")
             .add("rate_limit_group")
-            .add("queue_affinity");
+            .add("queue_affinity")
+            .add("backing_files");
         parser.parse(disk).map_err(Error::ParseDisk)?;
 
         let path = parser.get("path").map(PathBuf::from);
@@ -1203,6 +1204,12 @@ impl DiskConfig {
                     })
                     .collect()
             });
+        let backing_files = parser
+            .convert::<Toggle>("backing_files")
+            .map_err(Error::ParseDisk)?
+            .unwrap_or(Toggle(false))
+            .0;
+
         let bw_tb_config = if bw_size != 0 && bw_refill_time != 0 {
             Some(TokenBucketConfig {
                 size: bw_size,
@@ -1247,6 +1254,7 @@ impl DiskConfig {
             pci_segment,
             serial,
             queue_affinity,
+            backing_files,
         })
     }
 
@@ -3414,6 +3422,7 @@ mod unit_tests {
             pci_segment: 0,
             serial: None,
             queue_affinity: None,
+            backing_files: false,
         }
     }