libblkid: improve identification of ISO9660 partition

libblkid: improve identification of ISO9660 partition

Recent changes to the iso9660 handler attempt to better handle the case
where the media has both an ordinary ISO9660 filesystem, and a partition
table with a partition entry pointing at the ISO9660 filesystem.

Rather than assuming the presence of a partition table means that there
is an ISO9660 partition, check that such a partition exists before
deciding upon which device the metadata should be presented. One real
world example of this is the grub-mkrescue image format; add a unit test
to cover that.

Secondly, even if we find an appropriate entry in the partition table,
verify that we actually have a device that corresponds to the partition
where we would then proceed to expose this metadata. In the case of a
CD/DVD (i.e. /dev/sr0) we would not normally expect to see devices
corresponding to the partitions, in that case the ISO metadata should be
presented on the whole disk (/dev/sr0).

Fixes: 7ef86a0891 ("libblkid: improve handling of ISO files with partition tables")
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1783066
Signed-off-by: Daniel Drake <drake@endlessm.com>