The safest way to virtualize a Domain Controller is to add a new (virtual) DC to the network and promote it.
http://vknowledge.wordpress.com/2010/11/03/how-to-p2v-a-domain-controller/
MS System Center Virtual Machine Manager (SCVMM)
MS System Center 2012
Download from: http://technet.microsoft.com/en-us/sysinternals/ee656415.aspx
Read more at: http://www.petri.co.il/physical-to-virtual-with-disk2vhd.htm