If Vdsm is asked to set defaultRoute=True where gateway is missing, Vdsm silently fails. It does not report back defaultRoute=True, but it does not allow to set it on another network before clearing it first.
Verified on - vdsm-4.20.11-1.el7ev.x86_64 and 4.2.1-0.2.el7
This bugzilla is included in oVirt 4.2.1 release, published on Feb 12th 2018. Since the problem described in this bug report should be resolved in oVirt 4.2.1 release, it has been closed with a resolution of CURRENT RELEASE. If the solution does not work for you, please open a new bug report.