block/blkio: fall back on using path when fd setting fails

qemu_open() fails if called with an unix domain socket in this way:
    -blockdev node-name=drive0,driver=virtio-blk-vhost-user,path=vhost-user-blk.sock,cache.direct=on: Could not open 'vhost-user-blk.sock': No such device or address

Since virtio-blk-vhost-user does not support fd passing, let`s always fall back
on using `path` if we fail the fd passing.

Fixes: cad2ccc ("block/blkio: use qemu_open() to support fd passing for virtio-blk")
Reported-by: Qing Wang <qinwang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20230727161020.84213-4-sgarzare@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/blkio.c

@@ -711,19 +711,19 @@ static int blkio_virtio_blk_connect(BlockDriverState *bs, QDict *options,
          * In order to open the device read-only, we are using the `read-only`
          * property of the libblkio driver in blkio_file_open().
          */
-        fd = qemu_open(path, O_RDWR, errp);
+        fd = qemu_open(path, O_RDWR, NULL);
         if (fd < 0) {
-            return -EINVAL;
+            fd_supported = false;
+        } else {
+            ret = blkio_set_int(s->blkio, "fd", fd);
+            if (ret < 0) {
+                fd_supported = false;
+                qemu_close(fd);
+            }
         }
+    }
 
-        ret = blkio_set_int(s->blkio, "fd", fd);
-        if (ret < 0) {
-            error_setg_errno(errp, -ret, "failed to set fd: %s",
-                             blkio_get_error_msg());
-            qemu_close(fd);
-            return ret;
-        }
-    } else {
+    if (!fd_supported) {
         ret = blkio_set_str(s->blkio, "path", path);
         if (ret < 0) {
             error_setg_errno(errp, -ret, "failed to set path: %s",