vsock_test: wait for the remote to close the connection

Before check if a send returns -EPIPE, we need to make sure the
connection is closed.
To do that, we use epoll API to wait EPOLLRDHUP or EPOLLHUP events
on the socket.

Reported-by: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>