Refactor vm_helper XML parsing code

There’s a lot of code duplication, and awkwardness going on there. Some helper(s) in the spirit of iptables_chains_parse() from features/step_definitions/tor.rb would be nice.

Feature Branch: wip/refactor_update_with_rexml_document

Parent Task: #10237

Original created by @anonym on 10864 (Redmine)

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