iothread: rename PollParamInfo to IOThreadParamInfo

iothread: rename PollParamInfo to IOThreadParamInfo

Commit 1793ad0247 ("iothread: add aio-max-batch parameter") added
a new parameter (aio-max-batch) to IOThread and used PollParamInfo
structure to handle it.

Since it is not a parameter of the polling mechanism, we rename the
structure to a more generic IOThreadParamInfo.

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: 20210727145936.147032-2-sgarzare@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>