Description of problem:

According to [1] a network attachment within a host can be created, modified and removed using the corresponding tags:

Create or modify -> <modified_network_attachments> 
Remove: -> <removed_network_attachments>

However it doesn't work when modifying or removing an existing network attachment.

[1] http://ovirt.github.io/ovirt-engine-api-model/4.2/#services/host/methods/setup_networks

Version-Release number of selected component (if applicable):
4.2

How reproducible:
Always

Steps to Reproduce:
1. Create a network and bind it to an interface as described at [1]:
~~~
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<action>
  <modified_network_attachments>
    <network_attachment>
      <network>
        <name>mynetwork</name>
      </network>
      <host_nic>
        <name>eth0</name>
      </host_nic>
    </network_attachment>
  </modified_network_attachments>
</action>
~~~

2. Try to modify the network to bind it to another interface:
~~~
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<action>
  <modified_network_attachments>
    <network_attachment>
      <network>
        <name>mynetwork</name>
      </network>
      <host_nic>
        <name>eth1</name>
      </host_nic>
    </network_attachment>
  </modified_network_attachments>
</action>
~~~

3. Or try to remove the network attachment:
~~~
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<action>
  <removed_network_attachments>
    <network_attachment>
      <network>
        <name>mynetwork</name>
      </network>
      <host_nic>
        <name>eth0</name>
      </host_nic>
    </network_attachment>
  </removed_network_attachments>
</action>
~~~

Actual results:

The network attachment cannot be modified failing with:
~~~
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<fault>
    <detail>[Cannot setup Networks. Network mynetwork is already attached via attachment 11111111-1111-1111-111111111111. A new attachment cannot be used for the same network, please reuse the existing one.]</detail>
    <reason>Operation Failed</reason>
</fault>

~~~

The network attachment cannot be removed failing with:
~~~
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<fault>
    <detail>[Cannot setup Networks. Cannot remove following Network Attachments, because they do not exist: 	null.]</detail>
    <reason>Operation Failed</reason>
</fault>
~~~

Expected results:

The network attachment successfuly modified or removed.

Additional info:

It's only possible to modify or remove a network attachment if it is referenced by ID.
The below examples would work.

To modify the network attachment:
~~~
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<action>
  <modified_network_attachments>
    <network_attachment id="11111111-1111-1111-111111111111">
      <network>
        <name>mynetwork</name>
      </network>
      <host_nic>
        <name>eth1</name>
      </host_nic>
    </network_attachment>
  </modified_network_attachments>
</action>
~~~

To remove the network attachment:
~~~
<action>
  <modified_network_attachments>
    <network_attachment id="11111111-1111-1111-111111111111"/>
  </modified_network_attachments>
</action>
~~~