I tried to use qemu-nbd in the shell script, but it seems that qemu-nbd has some delay.
Description of problem:

Steps to reproduce:
1.
```
cat ~/test.sh
#!/bin/bash
qemu-nbd -c /dev/nbd0 $1
mount -t ntfs3 -o uid=1000,gid=1000 /dev/disk/by-label/OS /mnt/OS
```
2.
```
sudo ~/test.sh ~/VM/win7_i386.qcow2
mount: /mnt/OS: special device /dev/disk/by-label/OS does not exist.
       dmesg(1) may have more information after failed mount system call.

```
Additional information:
But when I added a one-second delay between qemu-nbd and mount commands, the problem was solved.

The qemu-img convert command also has a similar problem. It seems that these commands have a certain delay. Is this in line with expectations?