Convert any image that is not a raw image, e.g. a qcow2, to a raw
image and open the partitions via loop devices. This replaces the
usage of nbd, which was racy and flaky.
Instead on relying on the kernel for the partition parsing, this
is now done manually via loop devices and start + offset taken
from sfdisk. As a result the read_partition function has been
adapted to be called at later time, after the partitions were
opened via loop devices.
Not using nbd also means that the partition table is not scanned
by the kernel anymore and udev is not triggered. As a result the
'PARTUUID' property is not present for dos/mbr partition layouts,
since it is auto-generated by udev/blkid. Relevant blkid files
and functions are:
blkid_partition_gen_uuid(par)
called from probe_dos_pt()
in file libblkid/src/partitions/dos.c line 295
defined in libblkid/src/partitions/partitions.c line 1374
which generates the uuid via snprintf using the format:
'"%.33s-%02x", par->tab->id, par->partno'
Based on https://github.com/karelzak/util-linux at ce8985cc7
NB: the loop device code is imported from osbuild, making this
tool depend on osbuild's private library.
NB: As of the image conversion, more disk space is required to
examine non-raw images.
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