ci: harden s390x qemu test against modloop/disk flakes

The s390x netboot initramfs ships virtio_net but not virtio_blk; the
block driver only becomes available after the modloop image is
downloaded over the network at boot. When that download fails, sshd
still comes up but /dev/vda never appears and setup-disk dies with
"/dev/vda is not a block device suitable for partitioning".

- Wait for /sys/block/vda/device before running setup-alpine,
  restarting the modloop and hwdrivers services on each retry.
- Log the QEMU console (screen -L) for both VM launches and dump the
  logs at the end of the job, so failures of the second boot (the
  other recurring flake, where ssh never comes back after reboot)
  are debuggable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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