migration_boot: Fix boot elements issue

[cliping]

per-device boot elements cannot be used together with os/boot elements. So update case.

Before:
Failed to define avocado-vt-vm1 for reason: error: Failed to define domain from /var/tmp/xml_utils_temp_98iqmrdd.xml error: unsupported configuration: per-device boot elements cannot be used together with os/boot elements

After:
(1/1) type_specific.io-github-autotest-libvirt.guest_os_booting.migration.boot.os_dev: PASS (154.95 s)
RESULTS : PASS 1 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML : /var/log/avocado/job-results/job-2026-01-26T21.49-10f7f4e/results.html
JOB TIME : 157.82 s

Summary by CodeRabbit

Release Notes

• Tests
  • Enhanced migration boot testing reliability with improved error handling for XML element operations during OS attribute configuration.

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Walkthrough

The change adds a protective try/except block in migration_boot.py to remove the boot disk XML node from the VM configuration. When os_attrs_boots is provided, the code attempts to remove the boot element at /devices/disk/boot from the XML after OS attributes are set. The exception handling gracefully manages cases where the element is absent or the underlying object doesn't support removal operations.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'migration_boot: Fix boot elements issue' is directly related to the main change in the PR, which removes boot disk XML nodes to resolve a conflict between per-device and os/boot elements.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing touches

• 📝 Generate docstrings

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