virtio-devices: Remove unused update_memory() from VirtioDevice trait

Now that virtio devices can be updated with add_memory_region(), there's
no need to keep update_memory() around.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

virtio-devices/src/device.rs

@@ -139,10 +139,6 @@ pub trait VirtioDevice: Send {
     /// after a shutdown() can lead to unpredictable results.
     fn shutdown(&mut self) {}
 
-    fn update_memory(&mut self, _mem: &GuestMemoryMmap) -> std::result::Result<(), Error> {
-        Ok(())
-    }
-
     fn add_memory_region(
         &mut self,
         _region: &Arc<GuestRegionMmap>,