block/raw-posix: implement .bdrv_detach/attach_aio_context()

Drop the assumption that we're using the main AioContext for Linux AIO.
Convert the Linux AIO event notifier to use aio_set_event_notifier().

The .bdrv_detach/attach_aio_context() interfaces also need to be
implemented to move the event notifier handler from the old to the new
AioContext.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/block/linux-aio.c b/block/linux-aio.c
index 53434e2..7ff3897 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -177,6 +177,20 @@
     return NULL;
 }
 
+void laio_detach_aio_context(void *s_, AioContext *old_context)
+{
+    struct qemu_laio_state *s = s_;
+
+    aio_set_event_notifier(old_context, &s->e, NULL);
+}
+
+void laio_attach_aio_context(void *s_, AioContext *new_context)
+{
+    struct qemu_laio_state *s = s_;
+
+    aio_set_event_notifier(new_context, &s->e, qemu_laio_completion_cb);
+}
+
 void *laio_init(void)
 {
     struct qemu_laio_state *s;
@@ -190,8 +204,6 @@
         goto out_close_efd;
     }
 
-    qemu_aio_set_event_notifier(&s->e, qemu_laio_completion_cb);
-
     return s;
 
 out_close_efd: