virtio-devices, vmm: Shutdown VMM on virtio thread panic

Shutdown the VMM in the virtio (or VMM side of vhost-user) thread
panics.

See: #3031

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

virtio-devices/src/vsock/mod.rs

@@ -275,6 +275,7 @@ mod tests {
                     TestBackend::new(),
                     false,
                     seccompiler::SeccompAction::Trap,
+                    EventFd::new(EFD_NONBLOCK).unwrap(),
                 )
                 .unwrap(),
             }