Description of problem:
Update affinity group drop general positive flag

Version-Release number of selected component (if applicable):
ovirt-engine-4.1.0-0.4.master.20170115141203.git30e806b.el7.centos.noarch

How reproducible:
Always

Steps to Reproduce:
1. Create positive, hard affinity group
<affinity_group>
  <name>test_2</name>
  <enforcing>true</enforcing>
  <hosts_rule>
    <enabled>true</enabled>
    <enforcing>false</enforcing>
    <positive>true</positive>
  </hosts_rule>
  <positive>true</positive>
  <vms_rule>
    <enabled>true</enabled>
    <enforcing>true</enforcing>
    <positive>true</positive>
  </vms_rule>
  <hosts />
  <vms />
</affinity_group>
2. Update name of the affinity group
<affinity_group>
  <name>test_3</name>
</affinity_group>
3.

Actual results:
<affinity_group href="/ovirt-engine/api/clusters/00000002-0002-0002-0002-00000000017a/affinitygroups/9b80dabe-ef53-4f5c-9793-f2c8d45582d6" id="9b80dabe-ef53-4f5c-9793-f2c8d45582d6">
  <name>test_3</name>
  <enforcing>true</enforcing>
  <hosts_rule>
    <enabled>true</enabled>
    <enforcing>false</enforcing>
    <positive>true</positive>
  </hosts_rule>
  <vms_rule>
    <enabled>false</enabled>
    <enforcing>true</enforcing>
    <positive>false</positive>
  </vms_rule>
  <hosts />
  <vms />
</affinity_group>
No general positive flag at all after update, VM to VM affinity dropped to the disable state
<vms_rule>
    <enabled>false</enabled>
...

Expected results:
Update succeeds but change only things that I specified under the PU request

Additional info:
The problem relevant only for the REST API