gre: Temporary workarounds for GRE on Xen.
Two issues exist that prevent GRE tunnels from working properly on
Xen: iptables rules and brcompatd. The first issue is that GRE
traffic is not permitted by the default iptables rules. The second
is that brcompatd removes ports from the configuration that it
believes do not exist, which causes GRE ports to be deleted before
they are created.
This commit avoids these issues though not in the most optimal way.
Better fixes will be forthcoming in the near future.