Description of problem:
Meaningless request

PUT /api/vms/{id}
<vm>
    <small_icon />
</vm>

is accepted.

Version-Release number of selected component (if applicable):
3.6 branch, commit 5ef7702

How reproducible:
100%

Steps to Reproduce:
1. Make following request as admin user
PUT /api/vms/{id}
<vm>
    <small_icon />
</vm>


Actual results:
200 ok

Expected results:
400 bad requst