Generify the fan mapping support and utilise that to implement fan
mappings over vxlan transport.

Expose the existance of this functionality (when the module is loaded)
via an additional sysctl marker.
Signed-off-by: Jay Vosburgh <jay.vosburgh@canonical.com>
[apw@canonical.com: added feature marker for fan over vxlan.]
Signed-off-by: Andy Whitcroft <apw@canonical.com>