xen_disk: treat "aio" as "raw"

Sometimes the toolstack uses "aio" without an additional format
identifier, in such cases use "raw".

Updated in v2:

- fix code style.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: NAlexander Graf <agraf@suse.de>

hw/xen_disk.c

@@ -633,6 +633,9 @@ static int blk_init(struct XenDevice *xendev)
             blkdev->filename  = blkdev->params;
         }
     }
+    if (!strcmp("aio", blkdev->fileproto)) {
+        blkdev->fileproto = "raw";
+    }
     if (blkdev->mode == NULL) {
         blkdev->mode = xenstore_read_be_str(&blkdev->xendev, "mode");
     }