network: allow guest to guest IPv6 without gateway definition

network: allow guest to guest IPv6 without gateway definition

This patch adds the capability for virtual guests to do IPv6
communication via a virtual network interface with no IPv6 (gateway)
addresses specified.  This capability has always been enabled by
default for IPv4, but disabled for IPv6 for security concerns, and
because it requires the ip6tables command to be operational (which
isn't the case on a system with the ipv6 module completely disabled).

This patch adds a new attribute "ipv6" at the toplevel of a <network>
object.  If ipv6='yes', the extra ip6tables rules required to permite
inter-guest communications are added when the network is started. If
it is 'no', or not present, those rules will not be added; thus the
default behavior doesn't change, so there should be no compatibility
issues with any existing installations.

Note that virtual guests cannot communication with the virtualization
host via this interface, because the following kernel tunable has
been set:

   net.ipv6.conf.<bridge_interface_name>.disable_ipv6 = 1

This assures that the bridge interface will not have an IPv6
link-local (fe80::) address.

To control this behavior so that it is not enabled by default, the parameter
ipv6='yes' on the <network> statement has been added.

Documentation related to this patch has been updated.
The network schema has also been updated.