Description of problem:
IPv6 only default route is supported via engine, but vdsm is still not capable
to report default route true for only IPv6 setup.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. Set only IPv6 static network with gateway, defaultRoute = True
2. Get networkcaps
defaultRoute in caps is False
defaultRoute in caps is True
Currently defaultRoute is in idrect relation with IPv4.
Let's check that this will no out-of-sync because of the described behavior in 4.4.