block: mark AioContext as recursive

AioContext can be accessed recursively, in fact that's what we do with
aio_poll.  Marking the GSource as recursive avoids that GLib blocks it
and unblocks it around every call to aio_dispatch, which is a pretty
expensive operation.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

async.c

@@ -300,6 +300,7 @@ AioContext *aio_context_new(Error **errp)
         error_setg_errno(errp, -ret, "Failed to initialize event notifier");
         return NULL;
     }
+    g_source_set_can_recurse(&ctx->source, true);
     aio_set_event_notifier(ctx, &ctx->notifier,
                            (EventNotifierHandler *)
                            event_notifier_test_and_clear);