Don't let naughty dhcp servers trick us into setting default routes.

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
CA-137892
Commit 60bcfef60947abf5980ed46c415f0a4d6c264db4 from xcp-networkd
backported to the xs64bit-ring3 branch of xen-api.
Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>