runtime: Fix virtiofs fd leak

The kata runtime invokes removeStaleVirtiofsShareMounts after
a container is stopped to clean up the stale virtiofs file caches.

Fixes: #6455
Signed-off-by: Feng Wang <fwang@confluent.io>

src/runtime/virtcontainers/container_test.go

@@ -302,6 +302,7 @@ func TestMountSharedDirMounts(t *testing.T) {
 	sandbox := &Sandbox{
 		ctx:        context.Background(),
 		id:         "foobar",
+		agent:      newMockAgent(),
 		hypervisor: &mockHypervisor{},
 		config: &SandboxConfig{
 			HypervisorConfig: HypervisorConfig{