List a targets sub-targets/children with Acquire (Improvement/Ease of use)

[lhaagsma]

Acquire is able to collect information from sub-targets (VMs) with the --children flag. It is also possible to specify a single child with --child. However for the various hypervisor platforms on which this is applicable the means to list and identify the right file to supply to --child is not straightforward.

Because Acquire already is able to identify a targets children, why not just easy expose these in a human friendly manner with a --list-children argument.

I will create a PR for this.