migration: Add destination VM backup and cleanup support

[hholoubk]

Add optional destination VM backup/cleanup functionality to MigrationBase:

• New parameter 'do_destination_vm_backup' (default: no) to enable feature
• Accept optional remote_virsh session for destination host operations
• Check if VM exists on destination before migration
• Fail test if destination VM is running (blocker condition)
• Backup and undefine pre-existing stopped VMs to avoid conflicts
• Use comprehensive undefine options (--nvram, --managed-save, etc.)
• Restore destination VM in cleanup_default()

Update migration_iommu_device test:

• Add setup_remote_virsh_session() helper function
• Create single remote virsh session shared across test operations
• Refactor check_iommu_xml() to reuse remote session (no duplication)
• Add cleanup_default() call in finally block
• Properly close remote session in finally block

This allows migration tests to run on non-clean environments without depending on destination host state.

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