iothread: use IOThreadParamInfo in iothread_[set|get]_param()

Commit 0445409d ("iothread: generalize
iothread_set_param/iothread_get_param") moved common code to set and
get IOThread parameters in two new functions.

These functions are called inside callbacks, so we don't need to use an
opaque pointer. Let's replace `void *opaque` parameter with
`IOThreadParamInfo *info`.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20210727145936.147032-3-sgarzare@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>