Seems that there is a delay before the disk can be used when using newer
QEMU versions (e.g. on Debian trixie). We get errors like these:
sfdisk: cannot open /dev/nbd0: Inappropriate ioctl for device
or
mount: /srv/strongswan-testing/build/loop: special device /dev/nbd0p1 does not exist.
A sleep before the next command seems to help.
Also see [1].
[1] https://gitlab.com/qemu-project/qemu/-/issues/1413