Add test for net-update

```
NAME
    net-update - update parts of an existing network's configuration

  SYNOPSIS
    net-update <network> <command> <section> <xml> [--parent-index <number>] [--config] [--live] [--current]

  OPTIONS
    [--network] <string>  network name or uuid
    [--command] <string>  type of update (add-first, add-last (add), delete, or modify)
    [--section] <string>  which section of network configuration to update
    [--xml] <string>  name of file containing xml (or, if it starts with '<', the complete xml element itself) to add/modify, or to be matched for search
    --parent-index <number>  which parent object to search through
    --config         affect next network startup
    --live           affect running network
    --current        affect current state of network

```
With my patch https://github.com/autotest/virt-test/pull/1724, you can use it here to get string of a section. 
```
bridge_string = network_xml.get_section_string(xpath="/bridge")
```
But be careful, xpath is the full path of element.