Instead of keeping the loop device of the base image and then opening
each partition as a loop device, remove the original loop device of the
base image and then create a loop device for each partition from the
file itself using the partition offsets.
The open_image() function is renamed to convert_image() and now only
handles converting qcow2 files to raw files if necessary.
The loop_open() context is done in analyse_image() instead, so that the
base loop device can be closed without removing the converted image.
This fixes the following issue with LVM partitions:
When the same lvm partition UUID is on two devices (e.g., /dev/loop0p4
and /dev/loop1), the 'vgchange -ay' command fails with the following
error:
Cannot activate LVs in VG rootvg while PVs appear on duplicate
devices.
This happens when we open the LVM partition as a separate loop device,
which we do for all partitions that we want to inspect.
NB: It's possible to restrict the vgchange command to a specific device
with --devices, but this isn't available in older versions of lvm2 (it
was introduced in 2.03.11).
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