The namespace sandbox spawn a process that set the namespaces
and wait for the for worker process to finish.

The worker process drops the `vhost::Listener` after the first
`accept()`. However, `util::wait_forchild()` never returns, so
the parent keeps all the FDs open, like the socket's FD in a
listen state. This is problematic because nothing prevents a
miss configured VMM to try to connect twice to the same socket
leaving the VMM waiting forever.

Let's close the socket FD in the parent before waiting for the
child to finish.

Signed-off-by: German Maglione <gmaglione@redhat.com>