Description of problem:
MAC addresses that has been re-assigned are not freed when detaching data domain.

Re-assigned MAC addresses are leaked in some way and on the next time we will detach the data domain this MAC addresses will not freed and will be considered in use by the engine. 

Version-Release number of selected component (if applicable):
4.1.0-0.4.master.20170109222652.git53fd6cb.el7.centos

How reproducible:
100%

Steps to Reproduce:
1. Create cluster with MAC pool range 00:00:00:00:00:10-00:00:00:00:00:11
2. Create new VM and add 1 vNIC from pool
3. Detach and remove data domain
4. Import data domain and the VM and re-assign the bad MAC(create situation in which this MAC will considered as bad, make it in use on destination or switch the clusters pool to make it out of range)
5. Now we have VM with MAC address that has been re-assigned
6. Detach and remove the data domain 
7. Import the data domain and the VM - the re-assigned mac from step 5^^ is now considered as in use, although we removed the VM on the detach data domain. The re-assigned mac from step 5^^ is now leaked.

Actual results:
re-assigned MAC address got leaked

Expected results:
MAC address should get freed on detach data domain

- See also BZ 1405761