virtio/vsock: use 'target' in notify_poll_in callback

This callback controls setting of POLLIN, POLLRDNORM output bits of poll()
syscall, but in some cases, it is incorrectly to set it, when socket has
at least 1 bytes of available data. Use 'target' which is already exists.

Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>