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
This does not modify the root image but uses the strongSwan version
installed there (avoids build dependencies on version installed on the
host to use pki to generate all the keys and certificates).