schema: Fix interface link state schema

schema: Fix interface link state schema

In commit edd1295e1da6bfe8e4e257e5fbfad71ac0bf7c87 I've introduced an
XML element that allows to configure state of the network interface
link. Somehow the RNG schema hunk ended up in a weird place in the
network schema definition. Move it to the right place and add a test
case.

Note that the link state is set up via the monitor at VM startup so I
originally didn't think of adding a test case.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1173468
(cherry picked from commit 8eb907b8d0fd73c8f88f9cd8df1fa0e7abf40c93)