vm-virtio: Implement reset() for virtio-vsock

The virtio specification defines a device can be reset, which was not
supported by this virtio-vsock implementation. The reason it is needed
is to support unbinding this device from the guest driver, and rebind
it to vfio-pci driver.

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

vm-virtio/src/vsock/mod.rs

@@ -307,7 +307,7 @@ mod tests {
                     queue_evts,
                     kill_evt: EventFd::new(EFD_NONBLOCK).unwrap(),
                     interrupt_cb,
-                    backend: TestBackend::new(),
+                    backend: Arc::new(RwLock::new(TestBackend::new())),
                 },
             }
         }