fix: do not panic if malformed snapshot file contains vhost devices

If a snapshot file is corrupted in a way that makes it _look_ like it
contains a vhost-user-blk device, restoring will hit an `unimplemented!`
inside `block::vhost_user/persist.rs`. So instead of using
`unimplemented!`, just unconditionally return an error here. These
errors are handled properly and cause Firecracker to exit gracefully
instead.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

src/vmm/src/devices/virtio/block/vhost_user/mod.rs

@@ -19,8 +19,8 @@ pub const QUEUE_SIZE: u16 = 256;
 pub enum VhostUserBlockError {
     /// Cannot create config
     Config,
-    /// Persistence error: {0}
-    Persist(crate::devices::virtio::persist::PersistError),
+    /// Snapshotting of vhost-user-blk devices is not supported
+    SnapshottingNotSupported,
     /// Vhost-user error: {0}
     VhostUser(VhostUserError),
     /// Vhost error: {0}

src/vmm/src/devices/virtio/block/vhost_user/persist.rs

@@ -38,6 +38,6 @@ impl Persist<'_> for VhostUserBlock {
         _constructor_args: Self::ConstructorArgs,
         _state: &Self::State,
     ) -> Result<Self, Self::Error> {
-        unimplemented!("VhostUserBlock does not support snapshotting yet");
+        Err(VhostUserBlockError::SnapshottingNotSupported)
     }
 }