iothread: generalize iothread_set_param/iothread_get_param

Changes in preparation for next patches where we add a new
parameter not related to the poll mechanism.

Let's add two new generic functions (iothread_set_param and
iothread_get_param) that we use to set and get IOThread
parameters.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20210721094211.69853-2-sgarzare@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>