aio / timers: Convert mainloop to use timeout

Convert mainloop to use timeout from default timerlist group
(i.e. the current 3 static timers)

main-loop.c produces a (possibly spurious) warning about
multiple iterations. Adapt the way this works for a signed
timeout and make the warning a bit safer.

Signed-off-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
