provide support for multiple identical tags and multi level xml output

[Tzur-i]

currently, pyxcli can't handle identical tags from xml output. it is best that if a tag already exists in the object that represents that XML, to always check if that tag exists, and if it does - add a new one instead of erasing it.

see an example in cpu_list command, and compare it with the output of cpu_list on the machine itself.