fix(v2k): stabilize libvirt cutover defaults for live migration

[dhslove]

Summary

This PR fixes a v2k cutover issue where KVM guests created from migrated VMs could fail live migration due to unstable libvirt/QEMU device defaults, especially around virtio-balloon.

It also refreshes the ablestack_v2k CLI help and CLI guide so defaults and examples match the current implementation more closely.

Changes

• disable memballoon in generated libvirt XML for v2k guests
• add explicit virtio-serial guest agent channel
• use a non-deprecated default video model
• stop forcing NIC link state to down in generated XML
• add a smoke-test case that verifies --compat-profile defaults to auto even when omitted
• refresh CLI --help output with:
  • default values for parameters that already have implementation defaults
  • command examples for each major command
  • notes for options accepted by cutover but not yet reflected in generated XML
  • fleet-style status --vm ... / --watch usage
• refresh docs/v2k/v2k_cli_guide.md to match the updated help and current behavior

Root Cause

v2k was generating a minimal libvirt domain XML and relying on libvirt defaults for several devices. In the failing case, live migration consistently failed while restoring virtio-balloon state on the destination host.

This PR removes the balloon device from generated v2k guest XML and makes the resulting domain definition more predictable for migration.

Files Changed

• lib/v2k/target_libvirt.sh
• tests/v2k_compat_installer_runtime_smoke.sh
• bin/ablestack_v2k.sh
• docs/v2k/v2k_cli_guide.md

Validation

Validated in code by:

• reviewing generated XML logic against the failing migration logs
• adding a regression-style smoke-test path for implicit compat-profile=auto
• checking branch state against upstream/main

Environment limitation:

• bash-based test execution was not run in this session due to local shell/runtime restrictions