fix(vm): fix rootless upgrade migration for VMs with container disks

[loktev-d]

Description

Fixes live migration of container-disk VMs in kubevirt:

1. unknown user deckhouse: a 1.9 virt-handler probing a 1.8 launcher's container disk via virt-chroot --user deckhouse fails (the old launcher image has no deckhouse user), so the source VMI never syncs. GetImageInfo now falls back to the legacy qemu user when the launcher predates deckhouse (pkg/virt-handler/container-disk/hotplug.go).
2. Hotplug container disk not mounted on the migration target: findVirtlauncherUID matched the target launcher by its hotplug-disks dir, but a terminated pod left in ActivePods keeps that dir, so a lingering pod yields multiple matches and skips the bind-mount. It now keys off the launcher command socket (live pod only).
3. Blank qemu-img errors: some funcs used cmd.StderrPipe() + cmd.Output(). Wait() closed the pipe before stderr was read, so every failure logged with no reason. Switched to a captured bytes.Buffer.

Why do we need it, and what problem does it solve?

What is the expected result?

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: vm
type: fix
summary: fix rootless upgrade migration for VMs with container disks

[deckhouse-BOaTswain]

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[deckhouse-BOaTswain]

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The target step completed with status: failure.