List active network interface for vm:

[root@srv2 ~]# virsh domiflist vmkdc
Interface  Type       Source     Model       MAC
-------------------------------------------------------
vnet7      bridge     br400      virtio      52:54:00:c9:04:15

Generate new unique MAC-address:

[root@srv2 ~]# cat macgen.py 
#!/usr/bin/python
# macgen.py script to generate a MAC address for guests on Xen
#
import random
#
def randomMAC():
        mac = [ 0x00, 0x16, 0x3e,
                random.randint(0x00, 0x7f),
                random.randint(0x00, 0xff),
                random.randint(0x00, 0xff) ]
        return ':'.join(map(lambda x: "%02x" % x, mac))
#
print randomMAC()

Add new NIC to guest. Use random generated MAC address:

[root@srv2 ~]# virsh attach-interface --domain vmkdc --type bridge --source br300 --model virtio --mac 00:16:3e:23:3c:36 --config --live
Interface attached successfully

List network interfaces for VM:

[root@srv2 ~]# virsh domiflist vmkdc                           
Interface  Type       Source     Model       MAC
-------------------------------------------------------
vnet7      bridge     br400      virtio      52:54:00:c9:04:15
vnet8      bridge     br300      virtio      00:16:3e:23:3c:36