fstrim shrinks raw volumes on v0.6.8

[0xpolarzero]

MicroSandbox v0.6.8 pins msb-imago 0.1.1 and msb_krun_devices 0.1.25.

An EOF-reaching virtio discard reaches File::try_discard_by_truncate, which calls file.set_len(offset).

Repro: https://github.com/0xpolarzero/imago-tail-discard-repro

before=2147483648 after=2015371264 ext4=2147483648 delta=-132112384

The live guest still reports 2 GiB. After restart, ext4 cannot mount the shortened image.

Fix: use an Imago version that preserves logical length for discard and write-zeroes.

Imago issue: XanClic/imago#7
Fix PR: XanClic/imago#8