Description of problem:
When passing <custom_properties/> to update vm, the custom properties on the vm aren't removed.
Just when passing - <custom_properties> <custom_property/> </custom_properties>
the custom properties are removed.

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


How reproducible:


Steps to Reproduce:
1. Add custom properties to a vm.
2. Try to remove the custom properties by passing <custom_properties/> to update vm.

Actual results:
The vm still have custom properties.


Expected results:
The vm doesn't have custom properties.


Additional info: