aio: prepare for introducing GSource-based dispatch

This adds a GPollFD to each AioHandler.  It will then be possible to
attach these GPollFDs to a GSource, and from there to the main loop.
aio_wait examines the GPollFDs and avoids calling select() if any
is set (similar to what it does if bottom halves are available).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>