rbd: use BlockDriverState's AioContext

Drop the assumption that we're using the main AioContext.  Convert
qemu_bh_new() to aio_bh_new() and qemu_aio_wait() to aio_poll().

The .bdrv_detach_aio_context() and .bdrv_attach_aio_context() interfaces
are not needed since no fd handlers, timers, or BHs stay registered when
requests have been drained.

Cc: Josh Durgin <josh.durgin@inktank.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

block/rbd.c

@@ -555,7 +555,7 @@ static void qemu_rbd_aio_cancel(BlockDriverAIOCB *blockacb)
     acb->cancelled = 1;
 
     while (acb->status == -EINPROGRESS) {
-        qemu_aio_wait();
+        aio_poll(bdrv_get_aio_context(acb->common.bs), true);
     }
 
     qemu_aio_release(acb);
@@ -588,7 +588,8 @@ static void rbd_finish_aiocb(rbd_completion_t c, RADOSCB *rcb)
     rcb->ret = rbd_aio_get_return_value(c);
     rbd_aio_release(c);
 
-    acb->bh = qemu_bh_new(rbd_finish_bh, rcb);
+    acb->bh = aio_bh_new(bdrv_get_aio_context(acb->common.bs),
+                         rbd_finish_bh, rcb);
     qemu_bh_schedule(acb->bh);
 }