AioContext: take bottom halves into account when computing aio_poll timeout

Right now, QEMU invokes aio_bh_poll before the "poll" phase
of aio_poll.  It is simpler to do it afterwards and skip the
"poll" phase altogether when the OS-dependent parts of AioContext
are invoked from GSource.  This way, AioContext behaves more
similarly when used as a GSource vs. when used as stand-alone.

As a start, take bottom halves into account when computing the
poll timeout.  If a bottom half is ready, do a non-blocking
poll.  As a side effect, this makes idle bottom halves work
with aio_poll; an improvement, but not really an important
one since they are deprecated.

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

aio-posix.c

@@ -249,7 +249,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
     /* wait until next event */
     ret = qemu_poll_ns((GPollFD *)ctx->pollfds->data,
                          ctx->pollfds->len,
-                         blocking ? timerlistgroup_deadline_ns(&ctx->tlg) : 0);
+                         blocking ? aio_compute_timeout(ctx) : 0);
 
     /* if we have any readable fds, dispatch event */
     if (ret > 0) {