main-loop: drop rfds/wfds/xfds for good

Now that all *_fill() and *_poll() functions use GPollFD we no longer
need rfds/wfds/xfds or pollfds_from_select()/pollfds_to_select().

>From now on everything uses GPollFD.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 1361356113-11049-8-git-send-email-stefanha@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
