CA-126948: Adding utils call for generating a random MAC using the Xensource OUI. This will allow us to avoid using a '02' or '06' MAC.

Signed-off-by: Rob Dobson <rob@rdobson.co.uk>
CA-126948: switch to using a generated MAC rather than letting XAPI handle this.

There is a very small chance that we might duplicate MACs, but given the likelyhood,
particularly as the VMs are being created/destroyed all the time, I do not think this
is a concern that warrants the work required to track MAC usage.

Signed-off-by: Rob Dobson <rob@rdobson.co.uk>
Merge pull request #45 from rdobson/MAC-CA-126948

CA-126948: Restricting the MAC allocations to use the Xensource OUI