ipwrapper.addrFlush: flush only globally routable addresses

Link-local address must be kept for DHCPv6 support.
This patch makes iproute2 configurator play nice so
even after removing a network from an interface it is
still usable for DHCPv6.

Change-Id: I7f60f32a09d92ad7ee39bf343e4331bd3e43a14b
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Fixes: I5708c4db6f6d450536bd243a0475164b89f46f93
Reviewed-on: http://gerrit.ovirt.org/32168
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>