dma-helpers: dma_blk_io() cancel support

dma-helpers: dma_blk_io() cancel support

Attempting to cancel a dma_blk_io() request causes an abort(3):

  void bdrv_aio_cancel(BlockAIOCB *acb)
  {
      ...
      while (acb->refcnt > 1) {
          if (acb->aiocb_info->get_aio_context) {
              aio_poll(acb->aiocb_info->get_aio_context(acb), true);
          } else if (acb->bs) {
              aio_poll(bdrv_get_aio_context(acb->bs), true);
          } else {
              abort();
          }
      }
      ...
  }

This happens because DMAAIOCB->bs is NULL and
dma_aiocb_info.get_aio_context() is also NULL.

This patch trivially implements dma_aiocb_info.get_aio_context() by
fetching the DMAAIOCB->ctx field.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1466451417-27988-1-git-send-email-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>