Support IPv6 address in EVPN MAC advertisement routes

If a VM has both IPv4 and IPv6 addresses, agent should advertise the VM
MAC address twice - once with IPv4 address and once with IPv6 address.
CN evpn code already handles this scenario for the most part. Tweak the
xmpp message decode routine and add tests.

Change-Id: I52ac38520107e0c3025383bc5281545e422cc251
Partial-Bug: 1381418