cache: Call vm.Disconnect() when close vm

After previous commit, found that kata-proxy is not quit
when vmcache server is stopped by ctrl-c.
The cause is current kata-proxy is setsid when it exec.  It will
not get the signal ctrl-c.

Call vm.Disconnect() when close vm in cache factory to handle
this issue.

Fixes: kata-containers#1726

Signed-off-by: Hui Zhu <[email protected]>

virtcontainers/factory/cache/cache.go

@@ -63,6 +63,7 @@ func New(ctx context.Context, count uint, b base.FactoryBase) base.FactoryBase {
 				case <-closed:
 					c.removeFromVmm(vm)
 					vm.Stop()
+					vm.Disconnect()
 					c.wg.Done()
 					return
 				}