main-loop: switch POSIX glib integration to GPollFD

Convert glib file descriptor polling from rfds/wfds/xfds to GPollFD.

The Windows code still needs poll_fds[] and n_poll_fds but they can now
become local variables.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 1361356113-11049-4-git-send-email-stefanha@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
